Was trying to capture the idea of "when it bites you, soon it'll gain control of you". As also seen on Soul Collector and one or two other cards. Those generally have it only happen when they kill the creature though; I wanted one that works best when it doesn't kill the creature straight up.

Came out unpleasantly wordy. An alternate phrasing has this be 4/4 and replaces the first two abilities with "Whenever ~ blocks or becomes blocked by a creature, put a -1/-1 counter on that creature and prevent all combat damage ~ would deal this turn." But that takes up even more words, pushing this onto 10 lines rather than 9!

Is the -1/-1 counter as a reminder necessary? Could it just be "When this deals damage to a creature, gain control of that creature as long as you control ~"?

For that matter, this is quite strong on blocking as well as attacking, even as a 1/4 or a 2/4 it could be interesting, so maybe it doesn't need the first ability either?

Or maybe 4/4 "If this deals damage to a creature you may prevent that damage. If you do, gain control of that creature as long as you control ~"?

I was inspired by the kind of scene where a character gets bitten by one of these monsters, and the protagonists have a brief window (minutes) in which to try to save the bitten one, before their humanity and allegiance slips away. So the delay between combat and the control change was deliberate. But the -1/-1 counter might not be necessary. Your final wording is target interesting.

Interestingly, the zombie version of this wouldn't have "as long as you control ~". Killing the zombie that bit you doesn't stop you turning into a zombie. But vampires and werewolves often work this way.