Catti-brie of Mithral Hall

First strike, reach
Whenever Catti-brie attacks, put a +1/+1 counter on it for each Equipment attached to it.
{1}, Remove all +1/+1 counters from Catti-brie: It deals X damage to target attacking or blocking creature an opponent controls, where X is the number of counters removed this way.

Alchemy Horizons: Baldur's Gate

Forgotten Realms Commander

First strike, reach
Whenever Catti-brie of Mithral Hall attacks, put a +1/+1 counter on it for each Equipment attached to it.
{1}, Remove all +1/+1 counters from Catti-brie: It deals X damage to target attacking or blocking creature an opponent controls, where X is the number of counters removed this way.

If Catti-brie of Mithral Hall has no counters, you can still activate the last ability. This won't deal any damage, but it will count as targeting something for abilities like that of Fey Steed.

If a blocking creature is destroyed due to damage dealt this way, the creature it was blocking is still considered blocked. Unless the attacking creature has trample, it won't deal any damage to the player or planeswalker it is attacking.