No. If a spell/effect specifies that you may cast it without paying the mana cost, then you can't choose to pay mana instead. The "you may" part refers to the whole "cast it without paying its mana cost". There's a few niche circumstances where say you Mind's Desire into a Misthollow Griffin, you could choose to cast the Griffin for its normal cost, but that is separate from the "free cast" effect of Mind's Desire.
For any such effect where you don't pay mana to cast it, X is always 0, and you can't pay any sort of alternative costs like Kicker.
Edit: due to the wording on the reminder text, I'm thinking (1) this battle type doesn't trigger things that trigger on cards transforming or entering the battlefield transformed (unless "cast transformed" also means it "enters transformed"), and (2) this is another mechanic that gets hosed by Teferi, Time Raveler. I'll wait for a rules expert to chime in on both of these, though.
This is correct. For a permanent to "transform" it must stay on the battlefield throughout, it stays the same object for summoning sickness or being tapped. When a battle is defeated and triggers, it actually changes zones a few times (Battlefield->Exile->Stack->Battlefield) and is a new object. It wouldn't trigger something like Cult of the Waxing Moon. It does though "enter transformed" for anything that cares (could find/think of any).
And yes, Teferi does hose this because it specifies you "exile it, then cast it." Even if you used something like Vampire Hexmage to do it in your main phase, its still trying to cast during the resolution of the ability, which isn't a time you can cast a sorcery. If it said you could cast it until end of turn, or some other window, it could get around that. But since it doesn't, you need to cast it while the defeated trigger is resolving.
It looks like it still enters the battlefield under its owner's control, so a non-bo with Doubling Season. And if your opponent has Vorinclex, Monstrous Raider out, he makes your battles easier.
Yes, you will draw the card. Magus Lucea Kane makes an exact copy of the spell/ability, whatever value X is on the original, it will be the same on the copy. So if you activate Lucea, then cast Aberrant for X>=5, you'll draw 2 cards - assuming you don't cast/activate some other X spell/ability in between and use up Lucea's trigger.
Nothing about Xantcha's activated ability requires her to be on the battlefield for it to resolve, so the players will still draw their cards. And even if Xantcha isn't on the battlefield when the abilities resolve, the game will look for the last known controller to lose the 2 life, so Player 5 will still get shocked.
Its a bit different if Xantcha changes control while the ability is resolving, whoever ends up with the her will lose the life instead. For example, Player 5 has a Zedruu the Greathearted to activate instead of a kill spell, and gives it to Player 4, 4 will lose the life now (but still draw their card).
And for weirdness, if P5 instead cast Ephemerate on Xantcha, no matter who she winds up with afterwards, it would be a new game object, so the abilities on the stack for P2,3,4 would still cause P5 to lose life.
Something like Oracle of Dust that could remove All Hallow's Eve card from exile would be able to stop it post-casting, or Grafdigger's Cage type effects to stop it being useful, but I don't think a Stifle-type effect could ever do it in response to the upkeep trigger. Even Time Stop only delays a turn.
I thought that too, until I looked more closely halfway down the textboxes on Unholy Heat and Eldritch Evolution. They both have a little black border starting ~75% of the way down. I really didn't notice it until it was pointed out.
Borderless cards usually (always?) have those little curved bits at the bottom, and Planeswalker art fills the whole frame, even behind the text box. Borderless Planeswalkers just sorta combine the two ideas.
You choose the targets for the copies as the Storm of Forms cast trigger resolves, so if it isn't a valid target at that point, no copy can target it. In the first example, Wrexial still has Shroud at that point, and in the second example the Leviathan is still in exile, neither is valid.
There is a small window to respond to the cast trigger of Storm of Forms with something like a Disenchant to the Greaves/Banish, which could allow the copies to target something that the original Storm couldn't, but all of the copies should always have the same pool of targets to choose from.
Just like Phyrexian Obliterator the card might as well read "unblockable 5/5" 90% of the time, and that's fine.
Compared to Obliterator, this is much more "blockable" most of the time. Having trample makes Obliterator worse to deal with because even if you throw something in the way, you still take damage and have to sac more permanents, and if you try to block with enough creatures to soak up the trample, you're likely to clear your entire board as well. This Exemplar/Vindicator not only is possible to chump with small creatures, but as the ability only hits a single target, its "only" a 2-for-1 at worst, and at best quasi-trample based on your blocker's power.
It is still a helluva card, but I think its going to be a lot less brutal than the Obliterator.
edit - didn't think of the other side of the battle, I guess I would hate to attack into this guy.
Found some old threads about Overburden and similar issues. The takeaway seems to be this: the wording "Whenever a player puts" is distinct from ETB because it matters who is controlling the effect that puts the creature on the battlefield.
Roon of the Hidden Realm is the source of the flicker, whoever controls the Roon bounces a land, regardless of who controls the entering creature. Parallax Wave says "each player returns" so the entering creature's controller is the one that bounces the land.
Yet another old, weird functionality that they don't use anymore.
Not that its strictly relevant to the question, but the 2nd line of text on Questing Beast makes that block illegal. The same rules would apply to a 3/3 though.
Its easy to miss with QB's wall of text, or maybe your wife is just a big cheater at card games!
I mentioned already in another thread but, unless I am missing something, we are still lacking the cards from 29-36 in the commander section right after the set booster exclusive cards.
For any such effect where you don't pay mana to cast it, X is always 0, and you can't pay any sort of alternative costs like Kicker.
And yes, Teferi does hose this because it specifies you "exile it, then cast it." Even if you used something like Vampire Hexmage to do it in your main phase, its still trying to cast during the resolution of the ability, which isn't a time you can cast a sorcery. If it said you could cast it until end of turn, or some other window, it could get around that. But since it doesn't, you need to cast it while the defeated trigger is resolving.
Too bad they future-proofed Solemnity.
Its a bit different if Xantcha changes control while the ability is resolving, whoever ends up with the her will lose the life instead. For example, Player 5 has a Zedruu the Greathearted to activate instead of a kill spell, and gives it to Player 4, 4 will lose the life now (but still draw their card).
And for weirdness, if P5 instead cast Ephemerate on Xantcha, no matter who she winds up with afterwards, it would be a new game object, so the abilities on the stack for P2,3,4 would still cause P5 to lose life.
Ajani Goldmane
Basri Ket
Borderless cards usually (always?) have those little curved bits at the bottom, and Planeswalker art fills the whole frame, even behind the text box. Borderless Planeswalkers just sorta combine the two ideas.
There is a small window to respond to the cast trigger of Storm of Forms with something like a Disenchant to the Greaves/Banish, which could allow the copies to target something that the original Storm couldn't, but all of the copies should always have the same pool of targets to choose from.
Compared to Obliterator, this is much more "blockable" most of the time. Having trample makes Obliterator worse to deal with because even if you throw something in the way, you still take damage and have to sac more permanents, and if you try to block with enough creatures to soak up the trample, you're likely to clear your entire board as well. This Exemplar/Vindicator not only is possible to chump with small creatures, but as the ability only hits a single target, its "only" a 2-for-1 at worst, and at best quasi-trample based on your blocker's power.
It is still a helluva card, but I think its going to be a lot less brutal than the Obliterator.
edit - didn't think of the other side of the battle, I guess I would hate to attack into this guy.
https://www.mtgsalvation.com/forums/magic-fundamentals/magic-rulings/magic-rulings-archives/265018-overburden-parallax-wave
Found some old threads about Overburden and similar issues. The takeaway seems to be this: the wording "Whenever a player puts" is distinct from ETB because it matters who is controlling the effect that puts the creature on the battlefield.
Roon of the Hidden Realm is the source of the flicker, whoever controls the Roon bounces a land, regardless of who controls the entering creature.
Parallax Wave says "each player returns" so the entering creature's controller is the one that bounces the land.
Yet another old, weird functionality that they don't use anymore.
Its easy to miss with QB's wall of text, or maybe your wife is just a big cheater at card games!
Those are basics
Looks like is a Gatherer bug with the DMC set. Not actually legal, just something that is apparently too much for WotC to fix for 2+ weeks.