Goldspan Dragon's triggered ability won't trigger while it's a spell on the stack and becomes the target of another spell (such as Counterspell). This is because in general, an ability of an object (other than an instant or sorcery spell) doesn't work unless that object is on the battlefield (C.R. 113.6).
Neither does Goldspan Dragon's triggered ability trigger if Goldspan Dragon is countered.
In this situation you control two permanents, not just one, that have Anowon's triggered ability. And the corresponding ability of each one will trigger whenever "one or more Rogues you control deal combat damage to a player" (C.R. 603.2; see also C.R. 113.2c). This results in two abilities that go on the stack. And copying either ability will add a copy of that ability to the stack (C.R. 707.10).
After Rally the Ancestors returns creature cards in this scenario, you will control two legendary creatures named Teysa Karlov, two named Teysa, Orzhov Scion, and two nonlegendary creatures named Cruel Celebrant.
Assume that you control no permanents other than those mentioned and that the only creature cards in your graveyard are Teysa Karlov and Teysa, Orzhov Scion.
After Rally the Ancestors returns creature cards in this scenario, you will control two legendary creatures named Teysa Karlov, two named Teysa, Orzhov Scion, and one nonlegendary creature named Cruel Celebrant.
Then, due to the legend rule, one legendary creature you control named Teysa Karlov and one named Teysa, Orzhov Scion (both of which are black) will be sent to their owners' graveyards. This would cause—
the triggered ability of Teysa, Orzhov Scion that died to trigger for the Teysa Karlov that died,
the triggered ability of the surviving Teysa, Orzhov Scion that died to trigger for the Teysa Karlov that died,
that ability to trigger for the Teysa, Orzhov Scion that died,
Cruel Celebrant's ability to trigger for the Teysa Karlov that died, and
Cruel Celebrant's ability to trigger for the Teysa, Orzhov Scion that died,
but instead, due to each Teysa Karlov, those five abilities trigger two additional times each (for a total of fifteen abilities) (C.R. 700.4, 603.6c, 603.10, 603.10a).
As a result, if all the abilities that trigger this way resolve, you will have created nine Spirit tokens, each opponent will have lost 6 life, and you will have gained 6 life.
EDIT: Correction after comment 3 was posted.
EDIT: Add rule citations and make correctness edit after comment 5 was posted.
Bequeathal's triggered ability will trigger if you sacrifice Bequeathal and the enchanted creature at the same time.
This is because that ability is a leaves-the-battlefield ability, which looks at the state of the game immediately before the event of leaving the battlefield to determine whether it triggers (C.R. 700.4, 603.6c, 603.10, 603.10a). In this case, Bequeathal was attached to Grizzly Bears immediately before both permanents were sacrificed (C.R. 303.4m), so its triggered ability will trigger when Grizzly Bears dies this way.
If a player discards a commander from their hand (for any reason, including due to Wheel of Fortune), that commander goes to the graveyard (C.R. 701.8a), and the next time a player would get priority, that commander's owner may choose to move that commander from the graveyard to the command zone (as a state-based action) (C.R. 117.5, 903.9a). (This was a recent rule change in Core Set 2021.)
If a commander would go to a player's library (for any reason, including due to Timetwister), its owner may move it to the command zone instead (C.R. 903.9b).
See also Land Tax, whose ability has three ways that a player need not find exactly "three basic land cards": "you may", "up to three", and C.R. 701.19b (see also this thread).
Headless Rider says "Whenever Headless Rider or another nontoken Zombie you control dies, ..."
Morbid Opportunist says "Whenever one or more other creatures die, draw a card. This ability triggers only once each turn."
If Headless Rider dies at the same time as one or more other "nontoken Zombie(s) you control", its ability will trigger not just once for each such Zombie but also once for Headless Rider itself (C.R. 603.3c). This is because the ability watches not just whether other "nontoken Zombie(s) you control" die, but also whether Headless Rider itself dies.
For the same reason, if Headless Rider dies alone, its ability will trigger.
If Morbid Opportunist dies at the same time as one or more other creatures, its ability will trigger only once (C.R. 603.3c, 700.1), and only if the ability didn't already trigger this turn.
If Morbid Opportunist dies alone, its ability won't trigger, even if didn't already trigger this turn.
There is no card with the exact text "Whenever another creature dies, put a +1/+1 counter on target creature" at the time of this writing. However, if Ob Nixilis, Unshackled dies at the same time as one or more other creatures, its last ability will trigger once for each such other creature, but if Ob Nixilis dies alone, that ability won't trigger.
Now, for the sake of curiosity: Is there a double-dip case where not only the effect copies an instant/sorcery, but also has the line "you may cast the copy" in it?
Assuming you mean an effect that copies an instant or sorcery spell and has a player cast the copy: No. Since the copy of the spell will be put on the stack (C.R. 707.10), casting it is unnecessary (review C.R. 601.2). (Note that copying a spell is not the same as casting it [C.R. 707.10].)
Is that the same ruling with Kefnet, where the "action of copying" targets a card in hand instead of an instant/sorcery on stack, but when you cast the copy you trigger Magecraft.
For purposes of an ability of the form "Whenever you cast or copy an instant or sorcery spell, ...", casting a copy of a card counts as casting a spell (C.R. 112.1b). Thus, such an ability will trigger if "you cast" a copy of a card with God-Eternal Kefnet (or Mizzix's Mastery) and the resulting spell is an instant or sorcery spell. (However, copying a card in a zone other than the stack is not the same as copying a spell [review C.R. 112.1, which defines spells].)
Note that Kefnet's second ability doesn't target anything (C.R. 115.1d), and that the magecraft ability word, like all ability words, has no game mechanics inherent to it (C.R. 207.2c).
Your opponent will eventually get priority to pay 2 to ignore Leonin Arbiter's ability's effect.
The option a player has to pay 2 to ignore that effect is a special action that can be taken any time that player has priority (C.R. 116.2d). Notice that after Aether Vial's activated ability resolves, every player will get priority before a spell or ability resolves again (such as Gifts Ungiven or Windswept Heath's ability) (C.R. 117.3b, 117.3d, 117.4).
With Sunbird's Invocation, after you cast a spell, if any (such as Creative Technique), you immediately put "the rest [of the revealed cards] on the bottom of your library in a random order" (C.R. 608.2c). The spell you cast doesn't resolve before you put those cards there this way. Indeed, no spell can resolve while another spell or ability is resolving.
No; when cards are put on the bottom of a library with Proteus Staff, "[t]hat library's owner doesn't reveal the order in which the cards go into the library" (C.R. 401.4).
If I had paid the ETB cost twice, and only one of the chosen targets had become illegal in response, how many +1/+1 counters would be placed on Spectral Adversary?
Two, assuming there are exactly two targets. If the "When you pay this cost..." ability has two or more targets, and not all of them are illegal, then when it resolves "you put" a number of +1/+1 counters on Spectral Adversary equal to the number of times "you paid" the cost referred to (not necessarily the number of targets or the number of legal targets), then the legal targets phase out (C.R. 608.2b; see also C.R. 609.3).
Here, where Spectral Adversary says "that many" (each place it appears), it refers to the number of times "you paid" 1U when its enters-the-battlefield ability resolves, not necessarily the number of targets or the number of legal targets of the "When you pay this cost..." ability (but to be clear, only legal targets will phase out due to the "When you pay this cost..." ability [C.R. 608.2b]).
Yes, and not only theoretically. Even in normal cases, a player's life total can become negative (C.R. 107.1b), but usually, a player with 0 or less life loses the game as a state-based action (C.R. 704.5a). However, if a permanent you control has the ability "You can't lose the game and your opponents can't win the game", you won't lose the game this way (C.R. 101.2).
Neither does Goldspan Dragon's triggered ability trigger if Goldspan Dragon is countered.
In this situation you control two permanents, not just one, that have Anowon's triggered ability. And the corresponding ability of each one will trigger whenever "one or more Rogues you control deal combat damage to a player" (C.R. 603.2; see also C.R. 113.2c). This results in two abilities that go on the stack. And copying either ability will add a copy of that ability to the stack (C.R. 707.10).
See also this thread and this thread.
After Rally the Ancestors returns creature cards in this scenario, you will control two legendary creatures named Teysa Karlov, two named Teysa, Orzhov Scion, and one nonlegendary creature named Cruel Celebrant.
Then, due to the legend rule, one legendary creature you control named Teysa Karlov and one named Teysa, Orzhov Scion (both of which are black) will be sent to their owners' graveyards. This would cause—
- the triggered ability of Teysa, Orzhov Scion that died to trigger for the Teysa Karlov that died,
- the triggered ability of the surviving Teysa, Orzhov Scion that died to trigger for the Teysa Karlov that died,
- that ability to trigger for the Teysa, Orzhov Scion that died,
- Cruel Celebrant's ability to trigger for the Teysa Karlov that died, and
- Cruel Celebrant's ability to trigger for the Teysa, Orzhov Scion that died,
but instead, due to each Teysa Karlov, those five abilities trigger two additional times each (for a total of fifteen abilities) (C.R. 700.4, 603.6c, 603.10, 603.10a).As a result, if all the abilities that trigger this way resolve, you will have created nine Spirit tokens, each opponent will have lost 6 life, and you will have gained 6 life.
EDIT: Correction after comment 3 was posted.
EDIT: Add rule citations and make correctness edit after comment 5 was posted.
This is because that ability is a leaves-the-battlefield ability, which looks at the state of the game immediately before the event of leaving the battlefield to determine whether it triggers (C.R. 700.4, 603.6c, 603.10, 603.10a). In this case, Bequeathal was attached to Grizzly Bears immediately before both permanents were sacrificed (C.R. 303.4m), so its triggered ability will trigger when Grizzly Bears dies this way.
If a commander would go to a player's library (for any reason, including due to Timetwister), its owner may move it to the command zone instead (C.R. 903.9b).
Morbid Opportunist says "Whenever one or more other creatures die, draw a card. This ability triggers only once each turn."
If Headless Rider dies at the same time as one or more other "nontoken Zombie(s) you control", its ability will trigger not just once for each such Zombie but also once for Headless Rider itself (C.R. 603.3c). This is because the ability watches not just whether other "nontoken Zombie(s) you control" die, but also whether Headless Rider itself dies.
For the same reason, if Headless Rider dies alone, its ability will trigger.
If Morbid Opportunist dies at the same time as one or more other creatures, its ability will trigger only once (C.R. 603.3c, 700.1), and only if the ability didn't already trigger this turn.
If Morbid Opportunist dies alone, its ability won't trigger, even if didn't already trigger this turn.
There is no card with the exact text "Whenever another creature dies, put a +1/+1 counter on target creature" at the time of this writing. However, if Ob Nixilis, Unshackled dies at the same time as one or more other creatures, its last ability will trigger once for each such other creature, but if Ob Nixilis dies alone, that ability won't trigger.
Note that Kefnet's second ability doesn't target anything (C.R. 115.1d), and that the magecraft ability word, like all ability words, has no game mechanics inherent to it (C.R. 207.2c).
The option a player has to pay 2 to ignore that effect is a special action that can be taken any time that player has priority (C.R. 116.2d). Notice that after Aether Vial's activated ability resolves, every player will get priority before a spell or ability resolves again (such as Gifts Ungiven or Windswept Heath's ability) (C.R. 117.3b, 117.3d, 117.4).
EDIT (Jan. 11): Clarification.
Here, where Spectral Adversary says "that many" (each place it appears), it refers to the number of times "you paid" 1U when its enters-the-battlefield ability resolves, not necessarily the number of targets or the number of legal targets of the "When you pay this cost..." ability (but to be clear, only legal targets will phase out due to the "When you pay this cost..." ability [C.R. 608.2b]).
See also this thread, and see also Grand Warlord Radha and Varina, Lich Queen.