Ooh. That could be a tough hole to crawl out of. I'm conflicted as to the power level of this card, which probably means it's just right.

I personally like how it can synergize with bounce effects.

I'm mostly convinced that the ability won't do much unless it's played when the opponent has one card left in hand. It's easy to keep an extra land in hand to protect the card that they actually want, and if the opponent is heckbent/hellbent then they probably won't hold a card in their hand with this on the field unless it's some sort of reactive removal/blowout card that they're banking on you walking into.

There's a lot to be said about the psychological pressure this would apply, however. Opponents may be constantly encouraged to make suboptimal plays (such as spending premium removal on this or not casting a second creature when they should, or spending mana on an on-board draw effect in response to the trigger) in order to not lose value.

Played at the right time this is gonna make people tear their hair out. But most games, it'll probably not do very much.

I like it. It's a nice variation on discard.

I suppose you could sandbag this until the opponent only has one card left in hand, but I feel like I'd rather just play Liliana's Specter at that point, since it's probably at least T5-6 and it has evasion.

Not necessary. Round 2 this, round 3 Mind Rot puts your opponent in a very awkward position. If you have any more discard to back that up, they could get stuck in Heckbent. They may think that it's better to play slowly to build their hand so that the worst case scenario doesn't happen. But that's exactly what a discard deck wants you to do.