If I have both Thassa, Deep-Dwelling and Purphoros, Bronze-Blooded on the battlefield, and I pay to use Purphoros, Bronze-Blooded's activated ability, in what order will the two abilities appear on the stack when the end step begins? They say "at the beginning of (your|the next) end step", respectively.
If I have both Thassa, Deep-Dwelling and Purphoros, Bronze-Blooded on the battlefield, and I pay to use Purphoros, Bronze-Blooded's activated ability, in what order will the two abilities appear on the stack when the end step begins? They say "at the beginning of (your|the next) end step", respectivly.
You control them both, so you choose the order.
603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose. (See rule 101.4.) Then the game once again checks for and resolves state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.
603.7e If an activated or triggered ability creates a delayed triggered ability, the source of that delayed triggered ability is the same as the source of that other ability. The controller of that delayed triggered ability is the player who controlled that other ability as it resolved.
Note that if Thassa's ability resolves first targeting Purphoros's doomed creature, the creature Thassa returns will not be subject to that sacrifice.
400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence. There are nine exceptions to this rule:
400.7a Effects from spells, activated abilities, and triggered abilities that change the characteristics or controller of a permanent spell on the stack continue to apply to the permanent that spell becomes.
400.7b Prevention effects that apply to damage from a permanent spell on the stack continue to apply to damage from the permanent that spell becomes.
400.7c An ability of a permanent can reference information about the spell that became that permanent as it resolved, including what costs were paid to cast that spell or what mana was spent to pay those costs.
400.7d Abilities that trigger when an object moves from one zone to another (for example, “When Rancor is put into a graveyard from the battlefield”) can find the new object that it became in the zone it moved to when the ability triggered, if that zone is a public zone.
400.7e Abilities of Auras that trigger when the enchanted permanent leaves the battlefield can find the new object that Aura became in its owner’s graveyard if it was put into that graveyard at the same time the enchanted permanent left the battlefield. It can also find the new object that Aura became in its owner’s graveyard as a result of being put there as a state-based action for not being attached to a permanent. (See rule 704.5m.)
400.7f If an effect grants a nonland card an ability that allows it to be cast, that ability will continue to apply to the new object that card became after it moved to the stack as a result of being cast this way.
400.7g If an effect allows a nonland card to be cast, other parts of that effect can find the new object that card becomes after it moves to the stack as a result of being cast this way.
400.7h If an effect causes an object to move to a public zone, other parts of that effect can find that object. If the cost of a spell or ability causes an object to move to a public zone, that spell or ability’s effects can find that object.
400.7i After resolving a madness triggered ability (see rule 702.34), if the exiled card wasn’t cast and was moved to a public zone, effects referencing the discarded card can find that object.
You control them both, so you choose the order.
Note that if Thassa's ability resolves first targeting Purphoros's doomed creature, the creature Thassa returns will not be subject to that sacrifice.