MTG Salvation

You indeed understand correctly. The card draw happens during the resolution of Baleful Mastery, and no players receive priority while a spell/ability is resolving. The next opportunity they have to cast Village Rites is after the Mastery has resolved, at which point their creature has already been exiled.

And to clarify further: if you play another Theater of Horrors, even if it’s physically the same one that was on the battlefield earlier, you won’t have access to cards exiled by the earlier iteration of Theater, as the new Theater of Horrors is a different object and unrelated to the one that had exiled those particular cards.

Targets for a spell or ability are chosen when that spell or ability goes on the stack, and can't be changed later (at least, not without another card like Bolt Bend). So, if you targeted Worldspine Wurm and it's exiled in response, you can't change the target of the ability and it'll do nothing when it tries to resolve.

Your "starting deck" refers to the deck you present at the start of each game during a match. So, if you sideboard in even CMC cards in game 2, you can't declare Obosh as your companion for that game.

You could, in theory, get even CMC cards from your sideboard into the game after it's started even if Obosh is your companion, because those cards weren't part of your starting deck.

You only control objects on the stack or the battlefield. Also, when it says a permanent you control, it specifically means a permanent on the battlefield.

Triomes can't be used in a 2-color Commander deck, due to this rule:

"903.5d A card with a basic land type may be included in a Commander deck only if each color of mana it could produce is included in the commander’s color identity.
Example: Wort, the Raidmother’s color identity is red and green. A Wort Commander deck may include land cards with the basic land types Mountain and/or Forest. It can’t include any land cards with the basic land types Plains, Island, or Swamp."

Edit: I should also point out that the rule making you produce colorless instead of colors outside of your commander's color identity no longer exists. For example, your City of Brass or Prismatic Vista can be used to produce any color of mana no matter the deck, not just the colors of your commaander's color identity. Useful for commanders that can cast off-color stuff, like Sen Triplets.

You didn't actually link a picture, but based on that text you seem to be asking about Commune with Lava. In that case:

When this spell resolves, you exile the top X cards of your library, where X is the value chosen when you cast the spell. For example, if you announced that X was 4 and you paid 4RR (4 generic mana and 2 red mana) for it, then when Commune with Lava resolves you exile the top 4 cards of your library. These 4 cards are placed in the exile zone face-up.

Now, until the end of your next turn, you may play those cards. To "play" a card means to play it if it's a land card, or cast it if it's a nonland card. As the spell doesn't say you can play without paying mana costs, you must still pay any mana costs to cast spells this way, as well as any additional costs the spells might have.

The spell also doesn't give you any special timing permission, so you can only cast the cards at the normal times you're allowed. For example, you can cast instants during your own turn or your opponent's turn, but sorceries/creatures/artifacts/enchantments/planeswalkers have to be cast during your own turn's main phase as normal. You can only play a land card exiled this way if you have land plays available. Under normal circumstances, this means you can only play one of the exiled lands during your turn, and only if yiou haven't played a land previously during the turn. Any cards played will go where they're normally supposed to go (sorceries and instants go to the graveyard, permanents go on the battleffield).

And finally, once you reach the end of your next turn, you no longer have the ability to play those exiled cards -- any cards exiled by Commune with Lava that you didn't play will remain in the exile zone, without you having a way to play them anymore. Hopefully this clears everything up for you; if not, let us know and we'll be glad to help with any further clarification you might need.

Threaten's target is a creature, regardless of whether that creature is tapped. If it's tapped, it'll become untapped by Threaten. If it's already untapped, then untapping it doesn't do anything to it, but you'll still gain control of the creature you targeted.

Yes. When your Illuna resolves and mutates on top or bottom of the non-Human you targeted, any and all "whenever this creature mutates" abilities will trigger. That includes Illuna's ability itself, as well as any abilities that were on the creature prior to Illuna mutating onto it.

Yes, that works how you want it. When there are multiple cost reductions to apply to casting a spell or activating an ability, you can apply them in any order you'd like. So, just have the reduction from Zirda apply, then Fluctuator, and you end up with a cycling cost of 0.

You're correct. Paradise Druid in your graveyard does not have hexproof, as it has a static ability that conditionally gives it hexproof while it's on the battlefield -- this is not the same as simply having the hexproof keyword.

You have to assign 1 damage to the creature, and the other 3 damage to the player. The damage is then increased as it's dealt, so the creature will take 3 damage (the excess damage doesn't then trample over, because you've already assigned the Super-Duper Death Ray's damage) and the player will take 5 damage.

There aren't really rules for how trample works on an instant, but that's how it works for trample damage assigned in combat, so I'd assume it work the same way here.

The legendary copies have to be dealt with when state-based actions are checked, which is before the triggers go on the stack. But Uro *did* enter the battlefield, so the abilities do indeed trigger. It's just that all of your Uro copies will be gone by the time the two triggers can actually go on the stack.

Nope. A creature with Morph can only be cast as a face-down creature. Isareth's ability doesn't let you cast the creature card in the graveyard; it just gets returned to the battlefield, so it will always be returned face-up.

Because Mirage Mirror did not enter the battlefield as a copy of Animate Dead, it won't get the ETB ability of Animate Dead to return the enchanted creature card to the battlefield. It will be an Aura with "enchant creature put onto the battlefield by Animate Dead," specifically referring to the Animate Dead-copy. Since there's nothing on the battlefield that fits that description, your Mirage Mirror-as-Animate Dead will be put into the graveyard the next time state-based actions are checked.
So, you can't stop your opponent's Worldgorger Dragon combo by having Mirage Mirror become a copy of Animate Dead.