It's similar to Vulture, in a way - but this requires proximity, which although it doesn't exist ingame (Flooding isn't true proximity, but play order), and would be tricky to coordinate properly in an active game, is an interesting idea. Until your eighth creature, you are limited to a net +2|+2 per creature death - which, although twice as powerful than a single Vulture, cannot be stacked on more than two creatures. At its best, this grants +6|+6, same as if you had six Vultures on the field, so I believe the unupped at least is balanced. In fact, I'd even recommend a cost reduction, especially due to its tricky nature. 3 unupped, 5-6 upped should be in the ballpark. I am unsure about the balance of the upped version, doubling the buff is something Vulture doesn't get, and having even one out is immediately like having four Vultures on the field.
You don't mention whether these stack, though I assume they don't - stacking would be a little much!
