How does Split Anvil interact with cards that cost X?
The Split Anvil artifact says:
When you play a spell, spells in hand that cost less are reduced to 0 ember for the turn.
I have a spell that costs X. If I cast it when I have 3 ember, will that be considered a spell that costs 3, and so will make Split Anvil apply to spells of cost 2 or less?
I just tested it and casting a spell that costs X is indeed considered to be cast using your current ember value, and so Split Anvil triggered and made everything else free.