That's just nasty. A deck is pretty much just screwed if you can drop this while they have no or very few creatures. Closest card I can find is Tainted Aether. Sure, this might be more removable, but with Tainted you can always sac your land at least to get your fattie to stick on the board.

Added the "discards a card" clause, so an opponent at least has a way to stick creatures onto the board. You're right though that this effect is very strong. It may still be too strong, but I'll leave it like this for now.

It's strong but nothing overwhelming in terms of power level. IMO the real concern is that it can create a very unfun scenario.

The added discard clause is a fine fix.

Oh, and this probably should say "sacrifices a creature unless he or she discards a card". Otherwise this doesn't do anything if the opponent has no cards in hand (except for the creature he or she casts), which IMO might be too weak (it does cost after all).

@Tahazzar: The rules don't agree with you. The creature's controller has to choose an action they can perform if able i. e. if they have a creature, but no cards, they have to choose the sacrifice option and vice versa.

It is only "unless" wording where on alternative is entirely optional (e. g. with your wording they could sacrifice the creature in response to the trigger and even if it was their only creature they wouldn't need to discard a card unless they chose to).

Worded as it is, it's going to cause arguments. "I choose discard a card, no cards, nothing happens" "But you can't do that!" "But it would SAY if I couldn't do that."

i.e. rules may like it, but reality won't. Since it can be fixed, do.

But; uh; don't because that then becomes an absolutely terribly boring lock-down again.

Long live Athreos, I guess.