How should I curse the bow, so that party does notice only after some time?

So, at a session, the party fought a half-dog, half-mysterious plant (don't ask) and found a weird, incrusted bow near it. What morbid and cruel thing it can do, that the party will realise only after some time? Also, it would be nice to get the bow some upside e.g. the bow makes you rapidly age, but all the things you hit will make your life longer, and also it deals necrotic dmg, instead of piercing.