Q: Merfolk Trickster makes creatures lose all abilities. How does that work with */* creatures like Majestic Myriarch?
A: If Merfolk Trickster is used on Majestic Myriarch, it wipes out the ability that normally defines the Myriarch's power and toughness. Without any ability to define its power and toughness, those */* stats are left to default to 0. Until now, all effects that caused a creature to lose all abilities also happened to set their power and toughness to a specific value, so this wouldn't matter, but this time we don't have that luxury—without some other effect boosting the Myriarch's toughness, it won't survive.
Why does this work differently than, say, Changeling? If you remove the Changeling ability the creature still has all creature types because Changeling applied in an earlier layer. But the CDA on Tarmogoyf or Majestic Myriarch are also applied in an earlier layer than the ability removing effect, why does it go back and reset the power and toughness after the ability is removed?
Effects that set power and/or toughness, including those from characteristic-defining abilities such as found in Majestic Myriarch and Tarmogoyf (C.R. 604.3), are part of layer 7 (power/toughness effects) (C.R. 613.1g, especially C.R. 613.3a), which is a later layer than layer 6 (which includes ability removing effects) (C.R. 613.1f). Thus, Majestic Myriarch would lose all abilities before its ability setting its power and toughness start to apply. This differs from changeling, which applies in layer 4 (which includes creature type changing effects) (C.R. 613.1d, 702.72a), which is an earlier layer than layer 6 (C.R. 613.5).
Characteristic-defining abilities that set power and toughness are applied first, in the power-and-toughness layer, after the layer where the ability is lost. So it doesn't apply.
Changeling is a characteristic-defining ability, but that has no particular primacy in applying the layer system. Mention of characteristic-defining abilities appears when computing dependency (CR 613.7a), but other than that, the main property of CDAs is the rule that says they apply in all zones and even outside the game.
Refer to CR 613.1.
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Why does this work differently than, say, Changeling? If you remove the Changeling ability the creature still has all creature types because Changeling applied in an earlier layer. But the CDA on Tarmogoyf or Majestic Myriarch are also applied in an earlier layer than the ability removing effect, why does it go back and reset the power and toughness after the ability is removed?
EDIT (Apr. 26): Add rule citation.
Changeling is a characteristic-defining ability, but that has no particular primacy in applying the layer system. Mention of characteristic-defining abilities appears when computing dependency (CR 613.7a), but other than that, the main property of CDAs is the rule that says they apply in all zones and even outside the game.
Refer to CR 613.1.
Awesome avatar provided by Krashbot @ [Epic Graphics].