MTG Salvation

Omnath, Locus of Mana

My opponent has untapped forests. I steal Omnath till end of turn. Is it possible for him to add mana into his manapool at the end of turn AFTER he regains Omnath? (Thus makes Omnath bigger and has extra mana ready for his next turn)

Normally, no. "Until end of turn" effects end in the cleanup step, where player usually don't get priority, which he needs to activate his mana abilities.

However, if a triggered ability triggers during the cleanup step, or a state based acton is performed, then players get priority during cleanup and another cleanup step will follow after this one. In that case, your opponent can activate his mana abilities and have the green mana "float" over to is turn due to Omnath, which he controls again at this point. Getting a state based action to be performed or a trigger to go off in the cleanup step is tricky but not that difficult, but how to do this is beyond the scope of the rules forum.

I assume you gain control of (not: steal) Omnath from your opponent, and your opponent's turn comes immediately after the current turn. In this case, your opponent will regain control of Omnath during the cleanup step (C.R. 514.2). However, in general, a player gets priority during this step (e.g., to activate abilities [C.R. 116.1b]) only "if any state-based actions would be performed and/or any triggered abilities are waiting to be put onto the stack (including those that trigger 'at the beginning of the next cleanup step')" right after the turn-based action in C.R. 514.2 (C.R. 514.3a, 514.3), a condition not present merely by giving Omnath's control back to your opponent.

Quote from Rezzahan »

Normally, no. "Until end of turn" effects end in the cleanup step, where player usually don't get priority, which he needs to activate his mana abilities.

However, if a triggered ability triggers during the cleanup step, or a state based acton is performed, then players get priority during cleanup and another cleanup step will follow after this one. In that case, your opponent can activate his mana abilities and have the green mana "float" over to is turn due to Omnath, which he controls again at this point. Getting a state based action to be performed or a trigger to go off in the cleanup step is tricky but not that difficult, but how to do this is beyond the scope of the rules forum.

So possible in theory, but no convenient example on how.

Well, I guess, one example won't hurt anybody.

Example:
A creature is buffed by Giant Growth or similar and gets a number of -1/-1 counters (Scar) that would otherwise reduce its toughness to 0 or less. When the "until end of turn" effect ends in cleanup, the creature's toughness is 0 or less, and a state based action is performed to put it into its owner's graveyard.

Also note that in the case that 'something happens' during Cleanup and players are allowed priority to do stuff, immediately after is a new Cleanup step.

Basically the game will continue till there's a 'clean' cleanup step. It's possible to chain many cleanup steps, but the game won't let the turn end till there's one without any shenanigans.

Quote from Rezzahan »

Well, I guess, one example won't hurt anybody.

Example:
A creature is buffed by Giant Growth or similar and gets a number of -1/-1 counters (Scar) that would otherwise reduce its toughness to 0 or less. When the "until end of turn" effect ends in cleanup, the creature's toughness is 0 or less, and a state based action is performed to put it into its owner's graveyard.

If, in this case, the said creature is Omnath and the buffing effect is simply green mana, opponent wouldn't have the chance to add mana to keep him alive prior to its death of -1/-1 counter, right?

Correct. When a state based action is performed, another round of checks is done right afterwards, until no SBAs are performed, then waiting triggers go on the stack, and only then does the active player get priority.

edit:
And also, SBAs are always checked before any player gets priority, so your opponent doesn't have a chance to produce green mana in the cleanup step, before Omnath dies. But if he can carry some over from a previous step/phase (like with Upwelling), he'll have green mana already to keep Omnath alive.

(While my original comment is correct in and of itself, it doesn't really adress the actual issue. Sorry about that, I was somewhat distracted when I wrote that "answer".)

Thanks all.