Hellcarver Demon

The triggered ability is mandatory. All the permanents you control, including all lands, will be sacrificed.

If you control two Hellcarver Demons that deal combat damage to a player at the same time, each Demon’s ability triggers. When the first one resolves, you’ll sacrifice all permanents except that Hellcarver Demon (meaning you’ll sacrifice the other one). Then you’ll exile six cards from your library and may cast nonland cards exiled that way. After those spells resolve, the other Demon’s ability resolves. You’ll sacrifice all permanents you now control (including the first Demon and any permanents put on the battlefield as a result of the spells you just cast), exile six more cards from your library, and cast any number of them.

The cards from your library are exiled face up.