If you have them, you can then use Stampede to deal damage on already-damaged enemy characters or Iago, Loud-Mouthed Parrot to force opponents into attacking you
The abilities of a creature can be determined by its rule text or the effect that brought it onto the battlefield in MTG
They can't choose to opt out and sacrifice all of their creatures
All in all, milling your opponent out isnt a particularly great strategy, and lets face it: there are much better cards youd want to play than Trepanation Blade
You have Madam Mim Fox who is the strongest of the Madam Mim cards to decimate your opponents board, Merlin Crab to add firepower to your units, Kit Cloudkicker who can bounce a lot of lower-powered characters, and even Peter Pan who can grant all of your Evasive characters Rush, enabling a lot of versatile plays
When Imperial Mask enters, it creates enchantment tokens that are copies of itself that it gives to each player