Whenever a creature deals damage to one or more players for the first time each turn, put a +1/+1 counter on it.
Whenever chaos ensues, each creature you control gains "{T}: This creature deals 1 damage to target player or planeswalker" until end of turn.
Each creature can have only one +1/+1 counter put on it this way per turn.
Unlike the Vampires in the _Innistrad_(TM) block, Stensia's ability triggers whenever a creature deals any damage, not just combat damage.