Madness at the Darkmoon Faire Cards List & Guide
PoisonousDeathrattle: Deal 1 damage to all enemies.
If you give this thing taunt, it breaks the AI. The AI knows not to kill it since their board will be wiped, but won't be able to react because of that.
I remember the days when playing Kel'Thuzad alongside a Target Dummy would break the AI.
It knew that it would just get resurrected, so it failed to see the point in killing it (even if they had like 30 damage on their board), so it wouldn't do anything until it drew a spell that could get rid of kel directly. A shame they fixed that