With the understanding that the Oracle text of Chaos Orb is different from the text you cited (C.R. 108.1), the ability that you give in comment 1 doesn't target anything (it lacks the word "target", for example) (C.R. 115.1c; see also C.R. 115.10a); thus if a player activates that ability, other players generally won't know which nontoken permanent will be chosen until that ability resolves (C.R. 608.2d). In general, however, if the player activating that ability does choose a nontoken permanent in advance, they will be bound to that choice unless another player intervenes (for situations other than sanctioned tournaments, see C.R. 722.2a-c).
Likewise, Chaos Orb's ability, as given in its Oracle text, doesn't target anything either (C.R. 115.1c, 108.1).
EDIT (Apr. 25): Edited, because some rules were renumbered in the meantime.
The spell put onto the stack with Ertai's Meddling is a different object from the one exiled with it as far as the game is concerned (C.R. 400.7), so that effects that say the original spell can't be countered (e.g., from Boseiju or Vexing Shusher's activated ability) won't necessarily carry over to the spell that's a copy of it (note that Boseiju says "...that spell can't be countered" and Vexing Shusher's activated ability says "Target spell can't be countered"). This is so even though the copy of the spell will have "all decisions made for" the original spell (C.R. 706.10).
Note that an effect that says a spell can't be countered is not an effect that can be carried over, without more, to a copy of that spell (C.R. 706.2; see also C.R. 706.10), as opposed to abilities like "This spell can't be countered" (as found in Vexing Shusher, for example [C.R. 108.1]), which can be carried over, in general, to a copy of that spell (C.R. 706.2, 207.1; see also C.R. 706.10, 109.3).
EDIT (Jun. 27): Correctness edit.
EDIT (Jul. 8, 2020): Deleted a clause for correctness.
In general, an ability of the form "At the beginning of [step or phase], if [condition], ..." triggers only at the beginning of the given step or phase and only if the given condition is true at that moment (C.R. 603.4, 603.2), and that ability will get to resolve only if that condition still holds upon resolving (C.R. 603.4, 608.2a). See also this thread. Certain choices have to be made when the ability goes on the stack, notably targets (see, e.g., Ana Sanctuary), but sacrificing a permanent is not one of them (no costs have to be paid to put a triggered ability on the stack) (C.R. 603.3, 603.3c-d, 601.2c-d).
Thus, if no opponent of yours controls three or more creatures by the time Defense of the Heart's ability (you control) would resolve, that ability does nothing (so that you don't sacrifice Defense of the Heart, search your library, etc.) (C.R. 608.2a). (A similar situation applies with Land Tax if no opponent of yours controls more lands than you by the time Land Tax's ability you control would resolve.) But if an opponent of yours does control three or more creatures when the Defense of the Heart ability would resolve, you follow that ability's instructions as much as you can (C.R. 608.2c, 101.3; see also C.R. 609.3), so that even if you no longer control Defense of the Heart when that ability resolves, you still search your library for two creature cards (and do the rest of the ability as much as possible) (C.R. 113.7a).
In general, in the unofficial Oathbreaker variant, if All Hallow's Eve is your signature spell, when that spell resolves, you exile All Hallow's Eve, but move it to the command zone instead of to exile, with two scream counters on it (C.R. 608.2c, 108.1; see also C.R. 614.6 and this thread). After that, however—
All Hallow's Eve's second ability won't trigger at the beginning of each of your upkeeps if All Hallow's Eve is in the command zone, since it will trigger only "if All Hallow's Eve is exiled", that is, is in exile, "with a scream counter on it", that is, with at least one scream counter on it (C.R. 603.2, 603.4, 108.1; see also this thread), and
if you then cast All Hallow's Eve from the command zone, the resulting spell will have no counters (in general, counters "on an object are not retained if that object moves from one zone to another" [C.R. 122.2]) — but even so, it won't necessarily make any creature cards return to the battlefield, since All Hallow's Eve's second ability isn't resolving.
Since in the unofficial Oathbreaker variant, it's not possible for a signature spell card to be in exile, there is generally no way, in that variant, for All Hallow's Eve's second ability to trigger (and have each player return all creature cards from their graveyard to the battlefield depending on the circumstances) (C.R. 113.6b, 113.6j, 603.4, 108.1).
See also Ertai's Meddling, which generates a triggered ability that won't trigger if the targeted spell was somehow moved to the command zone rather than to exile (C.R. 113.6b, 113.6j, 603.4, 108.1).
EDIT: Add rule citation after comment 7 was posted.
EDIT (Sep. 2): Some rules were renumbered with Core Set 2020.
EDIT (Sep. 10, 2020): Strike out certain text because of recent rule changes in the Commander variant.
Ninjutsu doesn't target a creature or anything else (notably, its expanded text lacks the word "target"), so that even a creature with hexproof or shroud can be chosen to return to its owner's hand due to ninjutsu as long as it's an "unblocked attacking creature you control" (C.R. 702.48a, 114.1c; review C.R. 702.11b, 702.18a; see also C.R. 114.10a, 114.1e).
When Scrounging Bandar's second ability resolves, if you move all +1/+1 counters from Scrounging Bandar onto another creature, and Scrounging Bandar's toughness thereby becomes 0 or less, Scrounging Bandar will go to the graveyard as a state-based action before any player gets priority to activate abilities, including Carrion Feeder's ability (C.R. 116.3b, 116.5, 704.5f, 613.1g, 613.3d). See also this thread.
OK, I didn't see anything specific about this, so I am asking. If Siege Rhino leaves play/battlefield/exile/etc for any reason and comes back to the battlefield later, its effect gets triggered, correct? So if I have a Siege Rhino on the battlefield and activate Planar Guide, when the Rhino returns, its effects will trigger, correct?
Siege Rhino's ability will trigger when it enters the battlefield, no matter how it got to the battlefield (C.R. 603.2). Compare Siege Rhino with Wild Pair or Flayer of the Hatebound, whose enters-the-battlefield abilities mention additional criteria to trigger (C.R. 603.2, 603.4). See also this thread.
So in short it cannot be a signature spell because it will never have an effect once it is returned to the command zone? Bummer, my playgroup doesn’t mind letting it resolve as normal, but my OCD doesn’t want to play something not technically legal.
Note that All Hallow's Eve does have an effect when it resolves, namely: "Exile All Hallow's Eve with two scream counters on it" (C.R. 608.2c, 108.1; under C.R. 113.3a, All Hallow's Eve's first ability is a spell ability). All Hallow's Eve can be cast like any other spell. The rules for the unofficial Oathbreaker variant do not limit signature spell cards to those that have an effect when they resolve.
EDIT (Sep. 2): One rule was renumbered with Core Set 2020.
If you control both Alms Collector and Spirit of the Labyrinth and an opponent of yours would draw two cards—
as a cost or as a choice (e.g., as found in Sylvan Library or Consecrated Sphinx), that player can't pay that cost or make that choice (C.R. 121.2b), so Alms Collector's effect can't replace that with you and that player each drawing one card instead. In general, if something is impossible, it can't be replaced (C.R. 614.7, 101.2, 101.3; see also C.R. 614.17c).
other than as a cost or choice (e.g., as found in Divination), and that opponent did not draw any cards yet this turn, the "(i)nstructio(n) to draw [two] cards [is] still ... partially carried out" (C.R. 121.2b, 101.3), so that the player draws only one card and Alms Collector's effect doesn't apply anymore.
other than as a cost or choice, and that opponent did draw one or more cards this turn, that player simply doesn't draw any cards, so that Alms Collector's effect doesn't apply anymore (C.R. 101.2).
All Hallow's Eve's second ability works only while All Hallow's Eve is in exile, not while it's in any other zone, including the command zone (C.R. 113.6b, 113.6j, 603.4, 108.1). (Note that if All Hallow's Eve would go to its owner's graveyard, but doesn't, when its second ability resolves — e.g., if its owner is enchanted with Wheel of Sun and Moon — each player will still "return all creature cards from their graveyard to the battlefield" [C.R. 614.6, 101.3, 108.1]: All Hallow's Eve doesn't say, for example, "...put it into your graveyard. If you do, each player returns..." [see also C.R. 118.12].)
Note that according to the rules for Oathbreaker (an unofficial variant of Magic), a signature spell card must go to the command zone instead of going anywhere else upon leaving the stack.
EDIT (Sep. 2): Some rules were renumbered with Core Set 2020.
EDIT (Dec. 23, 2020): Edited.
In general, an effect that says a permanent "enters the battlefield ... with" a given number of counters (C.R. 614.1c), or that says to "return" a card "to the battlefield ... with" a given number of counters, adds that many counters to that permanent as it enters the battlefield, whether the effect uses the word "additional" or not. (See also this thread and this thread.) Thus, for example, in this scenario, Triskelion or Tetravus will enter not only with the "noted number and kind of counters" for Tawnos's Coffin on it, but also with an additional three +1/+1 counters on it.
I should point out an important difference between Fatal Push and Spellstutter Sprite's triggered ability. The former can target any "creature" regardless of that creature's converted mana cost (C.R. 114.1a; see also this thread), whereas the latter can target only a "spell with converted mana cost X or less, where X is the number of Faeries you control" (C.R. 114.1d). Thus, for example, you can target a creature with, say, a converted mana cost of 5 as you cast Fatal Push, but you must target a spell with converted mana cost equal to or less than the number of Faeries you control as you put Spellstutter Sprite's triggered ability on the stack (C.R. 603.3d, 601.2c).
When Spellstutter Sprite enters the battlefield under your control and its triggered ability triggers this way, that ability counts the number of Faeries you control both when you put that ability on the stack and when that ability resolves, since at both moments the game checks whether a target has converted mana cost equal to or less than that number (C.R. 109.5, 603.3d, 601.2c, 608.2b).
Can one creature have multiple instances of "Exalted"? For example Quasali Pridemage paired up with Sublime Archangel?
In general, an object can have multiple instances of any triggered ability, including exalted, and each such instance triggers separately (C.R. 702.82a, 112.2c, 603.2; see also Maelstrom Wanderer [C.R. 702.84a]).
For the purposes of Chalice of the Void, neither activating a ninjutsu ability nor putting a card onto the battlefield is the same as casting a spell (review C.R. 702.48a; compare C.R. 701.2, activate, with C.R. 701.4, cast; see also this thread), so Chalice of the Void's triggered ability won't trigger from either action.
Likewise, Chaos Orb's ability, as given in its Oracle text, doesn't target anything either (C.R. 115.1c, 108.1).
EDIT (Apr. 25): Edited, because some rules were renumbered in the meantime.
Note that an effect that says a spell can't be countered is not an effect that can be carried over, without more, to a copy of that spell (C.R. 706.2; see also C.R. 706.10), as opposed to abilities like "This spell can't be countered" (as found in Vexing Shusher, for example [C.R. 108.1]), which can be carried over, in general, to a copy of that spell (C.R. 706.2, 207.1; see also C.R. 706.10, 109.3).
EDIT (Jun. 27): Correctness edit.
EDIT (Jul. 8, 2020): Deleted a clause for correctness.
Thus, if no opponent of yours controls three or more creatures by the time Defense of the Heart's ability (you control) would resolve, that ability does nothing (so that you don't sacrifice Defense of the Heart, search your library, etc.) (C.R. 608.2a). (A similar situation applies with Land Tax if no opponent of yours controls more lands than you by the time Land Tax's ability you control would resolve.) But if an opponent of yours does control three or more creatures when the Defense of the Heart ability would resolve, you follow that ability's instructions as much as you can (C.R. 608.2c, 101.3; see also C.R. 609.3), so that even if you no longer control Defense of the Heart when that ability resolves, you still search your library for two creature cards (and do the rest of the ability as much as possible) (C.R. 113.7a).
EDIT (Aug. 24): Add rule citation.
Since in the unofficial Oathbreaker variant, it's not possible for a signature spell card to be in exile, there is generally no way, in that variant, for All Hallow's Eve's second ability to trigger (and have each player return all creature cards from their graveyard to the battlefield depending on the circumstances) (C.R. 113.6b, 113.6j, 603.4, 108.1).
See also Ertai's Meddling, which generates a triggered ability that won't trigger if the targeted spell was somehow moved to the command zone rather than to exile (C.R. 113.6b, 113.6j, 603.4, 108.1).EDIT: Add rule citation after comment 7 was posted.
EDIT (Sep. 2): Some rules were renumbered with Core Set 2020.
EDIT (Sep. 10, 2020): Strike out certain text because of recent rule changes in the Commander variant.
EDIT (Sep. 2): One rule was renumbered with Core Set 2020.
See also this thread.
EDIT (Oct. 10, 2020): Correction. Some rules were renumbered in the meantime.
EDIT (Jul. 2, 2022): Update rule renumberings.
Note that according to the rules for Oathbreaker (an unofficial variant of Magic), a signature spell card must go to the command zone instead of going anywhere else upon leaving the stack.
EDIT (Sep. 2): Some rules were renumbered with Core Set 2020.
EDIT (Dec. 23, 2020): Edited.
EDIT (Jul. 1): Correction.