Right on the verge of "evil designs that don't work", I think this works, but it's clearly going to be confusing.

Or, "this deals damage to creatures in the form of +1/+1 counters on itself" :)

Actually, reminder text doesn't have to conform to rules and templating, so this could theoretically work. However, the actual rules surrounding it may be a bit complex.

Certainly fiddly. If it's in combat with a 2/2, it lives. If it fights a 2/2, it lives. If it uses Lavamancer's Skill to take down a half-damaged Disease Carriers, it lives.

If two of this gang-block a 4/4, at least one of them will live. If this deals first strike damage to something that then takes normal-strike damage and dies, this doesn't get the bonus. If this is in combat with a 1/3 and then something else pings the 1/3, this doesn't get the bonus. That... actually all seems quite reasonable...

In retrospect, "deal lethal damage" isn't well defined, and it should either count if it dealt damage to a creature which now has lethal damage on it, or if it dealt the last points of damage (possibly jointly with another creature). But hopefully that could be fixed up.

I posted this in CWNH not Evil Designs, because I think the idea was possibly workable as a vampire mechanic. But I think it's confusing to many players even if well designed: knowing what counts as lethal damage, knowing that if it does combat or fight, there's never a window where it dies before getting the bonus. I'm not sure if there's a way of phrasing it which is less confusing.

It was loosely inspired by Bandersnap :)