Would "At the beginning of your end step, if Cavalier was eligible to attack this turn but didn't, it deals 3 damage to you." work?> - 23 word-count as opposed to 27 - not much but it sounds cleaner.

I'd say "could have attacked this turn" rather than "was eligible to attack this turn".

But it might be simpler to just say "At end of combat on your turn, if ~ is untapped, ~ deals 3 damage to you". That works essentially the same (forces you to cast it in second main), and is arguably simpler.

The idea was an Erg Raiders but slightly better. I'll work on it, try to make it cleaner.