Without any special cards, my opponent lives with -19 damage after my attack to get to their turn
1) attacked with two celestial unicorns and opponent only blocks the one. The other deals lethal damage to take them down to -19.
2) My turn ends.
3) ACTUAL: Opponent's turn starts.
4) Opponent deals lethal damage down to -2, their life total is still -19.
5) They win?
EXPECTED: They die after taking lethal damage on my turn and do not get another turn.