i like this very much!
with +10/+10 it's OP, but maybe +8/+8 can be a good deal.
maybe, you can make its cost 7
|6
, and it applies only to life cards, but so with the forest spirit will become OP...
or it could poison the creature which it applies to for 6 poison tokens, so it would last no more than 3 turns (if the creature has >4 hp).
i like it!