MTG has introduced many incentives to exile cards, or rewards for cards that enter from exile
Glorybringer is expensive, but still not so expensive that it wont make sense to have access to at the top of an aggressive red decks curve
Between its access to menace, the fact this ability can be triggered off of any creature under its owner's control, and the fact that Commander's multiplayer nature increases the chance of an opponent not having a blocker, chances are this will be reliably easy to trigger
If the spell is no longer on the stack when Ancient Cellarspawn's last ability resolves, use its mana value as it last existed on the stack
Once a finality counter's in place, the creature it's on is exiled the next time it would die
This five-mana curve-topper punishes players for playing a variety of cards, including enchantments, while being a form of card advantage in its worst cases