MTG Salvation

Once the permanents referred to by Deputy of Detention's ability are exiled while Deputy of Detention is on the battlefield, those cards will return to the battlefield immediately after Deputy of Detention leaves it, even if Deputy of Detention has no abilities by then (C.R. 610.3, 610.3a).

Effects that add to the number of lands a player can play in a turn (such as found in Azusa, Fastbond, and Exploration) are governed by C.R. 305.2a-b. These rules are what makes this kind of effect special, and don't apply to other effects that allow a player to play cards of a certain kind in a manner not otherwise allowed (such as found in Muldrotha or Crucible of Worlds).

Rule of Law's ability could alternatively be read as "Each player who has cast a spell this turn can't cast additional spells" (compare with Ethersworn Canonist).

Quote from NavoMan »

So I could for example...play Zacama, Primal Calamity from my graveyard with Horde of Notions, tap all my lands for mana in response to Zacama's etb, untap all my lands, sacrifice Zacama to Phyrexian Altar, and repeat from the first step to generate infinite mana?

You can cause a loop like this.

I was also wondering if Horde of Notions' ability ignores timing restrictions, or not. Can I do this at instant speed, or do I need another card like Alchemist's Refuge, or Leyline of Anticipation to complete the combo?

The targeted Elemental card is played during the resolution of Horde of Notions's activated ability, which is a moment that cards normally can't be played (for spells, the rule is C.R. 608.2f). Compare that ability with Ornate Kanzashi's ability and see also this thread and this thread.

To play a nonland card means to cast that card as a spell (C.R. 701.13b). For the purposes of Zacama's triggered ability, if "you cast" Zacama with Horde of Notions and the permanent that spell becomes enters the battlefield, "you [have] cast" Zacama this way. This is the case even though Horde of Notions's activated ability allows a player to "play" a given card when it resolves.

Quote from VoidTimeWalker »

You can choose what order to pay costs in, so if you pay the sacrifice cost before the mana cost, you are no longer able to pay for mana wise.

No, that's not how it works. You activate mana abilities before you pay the cost of a spell with mana in its cost (C.R. 601.2f). In general, to pay for Heartfire, you—

• spend 1R, which is Heartfire's mana cost (C.R. 118.3a, 601.2f), and
• sacrifice a creature or planeswalker,

in either order of your choice (C.R. 601.2h).

If you attack with Angelic Guardian, it will count as a "creatur[e] you control" for the purposes of its triggered ability (C.R. 508.1k, 508.1m).

With two Expansions and Shock, a loop as I describe in this thread can result (although the loop I describe there involves Wild Ricochet, not Expansion, Wild Ricochet's feature of choosing new targets for the targeted spell is not relevant to the loop). And each time you copy, not just cast, Expansion (or any other instant or sorcery spell), Ral's first ability will trigger (not: proc). See also this thread.

In general, you can activate an activated mana ability (which includes tapping a land for mana) "whenever [you] have priority, whenever [you] are casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment" (C.R. 605.3a, 106.12). Thus, for example, if you have priority, you can tap a land for mana, then cast a spell or activate an ability (such as the first ability of Vizier of Tumbling Sands) without spending that mana to pay for that spell or ability.

While casting Heartfire, if that spell's "total cost includes a mana payment" as it normally does, you activate mana abilities (such as Faeburrow Elder's last ability) before you pay the spell's costs, including sacrificing a creature or planeswalker (C.R. 608.2f, 608.2h). Note that Faeburrow Elder's last ability doesn't care whether permanents you control are tapped or untapped.

Thus, if you tap Faeburrow Elder for mana while casting Heartfire, you add R if you control a red creature, even if you will then sacrifice that creature to pay for Heartfire.

Quote from RizyFontaine »

Ah, ok. So after the combat phase, the creatures will lose their buffs. So that means that they will still have their buffs during the combat phase.

Not exactly. Take the following scenario:

• The combat damage step starts. Elvish Archdruid is blocked by Grizzly Bears, and Llanowar Elves (2/2 thanks to Elvish Archdruid) is blocked by Zealous Guardian. Assume that none of these creatures were dealt damage yet this turn, and that Elvish Archdruid and Llanowar Elves are controlled by the same player.
• Grizzly Bears assigns damage equal to its power to Elvish Archdruid and vice versa. Zealous Guardian assigns damage equal to its power to Llanowar Elves and vice versa (C.R. 510.1).
• The combat damage assigned is now dealt (C.R. 510.2).
• Elvish Archdruid, Zealous Guardian, and Grizzly Bears have been dealt lethal damage, so all three creatures are destroyed as a state-based action (C.R. 510.3, 117.5, 704.5f). Now, since Elvish Archdruid isn't on the battlefield anymore, Llanowar Elves goes back to 1/1, so that the damage marked on Llanowar Elves is now lethal, so that Llanowar Elves is destroyed in another check for state-based actions (C.R. 117.5, 704.5f). Note that at this point, the game is still in the combat damage step.

Elvish Archdruid's first ability is a static ability; its effect giving "[o]ther Elf creatures you control ... +1/+1" lasts only while Elvish Archdruid is on the battlefield (C.R. 604.1, 611.3b, 113.6). Once Elvish Archdruid leaves the battlefield, that effect ends immediately and the power and toughness of creatures are adjusted accordingly.

Note that blockers are declared all at once, not one at a time (C.R. 509.1g).

Compare Gerrard with Academy Rector, which says "... you may exile [Academy Rector]. If you do, ..." (C.R. 108.1), so that the rest of the effect of Academy Rector's triggered ability is done only if Academy Rector is exiled this way (C.R. 118.12). This is unlike with Gerrard's triggered ability, where the action of exiling Gerrard is simply ignored if it can't be done (C.R. 101.3). See also this thread.

If an effect says a permanent "becomes" something (such as from Oko's second ability), the permanent affected by that effect remains the same permanent as before (see also C.R. 400.7 and this thread).

A creature can attack, and its activated abilities with the tap or untap symbol in their costs can be activated, only if that permanent was under its controller's control continuously since their most recent turn began (C.R. 302.6). This is true regardless of whether that permanent was a creature when that player's most recent turn began.

For example, if, during your turn—

• a noncreature artifact enters the battlefield under your control, then becomes a creature in the same turn, that permanent can't attack this turn while it's a creature.
• a noncreature artifact you controlled continuously since the turn began becomes a creature, that creature can attack as normal.

If Acidic Soil you control resolves, it normally deals as much damage to each player as lands they control. But if you also control Torbran, the damage dealt to each opponent this way is increased by 2, since Acidic Soil is red. (Note that Acidic Soil doesn't deal 1 damage to each player as many times as lands that player controls — check the card's Oracle text for the latest wording [C.R. 108.1].)

For example, say you and your opponent each control five lands. If Acidic Soil you control resolves while you control Torbran, it deals 5 damage to you and 7 damage to your opponent (5 damage plus 2 from Torbran, since Acidic Soil is red).

I want to note something about how spells and abilities that involve casting a card treat split cards.

Some spells and abilities say a player may "cast" a card with certain characteristics from a particular zone (examples include Electrodominance, Rishkar's Expertise, and Haakon, Stromgald Scourge, among others, but not cascade [C.R. 702.84a]). Since the recent rules update with Throne of Eldraine, for the purposes of determining whether that player can cast a split card this way, what matters are the characteristics of the half they choose (or the characteristics of both halves if that card is in that player's hand and has fuse, and the player so chooses) in that zone before that player begins to cast that card (C.R. 601.3e; C.R. 708.3a, 702.101a). For example, with Rishkar's Expertise, that player can cast from their hand—

• any half of a split card, if that half's converted mana cost is 5 or less, or
• both halves of a split card, if the card has fuse and their combined converted mana cost is 5 or less.

However, in general, if the spell or ability says "You may cast target ... card..." (as in Torrential Gearhulk) or "You may cast that card..." (as in cascade [C.R. 702.84a]), that spell or ability doesn't care about the card's characteristics at the moment the card starts to be cast (for abilities of the first kind, see C.R. 601.2c, 608.2b), so that if a player casts a split card this way, that player may choose either half of the card (or both halves if the card has fuse and is in the player's hand), even if the half or halves chosen don't meet the criteria called for by the spell or ability (C.R. 608.2f, 708.3a, 702.101a). For example, the sorcery half of an instant sorcery split card can be cast with Torrential Gearhulk this way (see also this thread).