Thoctar mortal

Deathbringer Thoctar

Whenever another creature dies, you may put a +1/+1 counter on Deathbringer Thoctar.
Remove a +1/+1 counter from Deathbringer Thoctar: It deals 1 damage to any target.

Commander 2013

Whenever another creature dies, you may put a +1/+1 counter on Deathbringer Thoctar.
Remove a +1/+1 counter from Deathbringer Thoctar: Deathbringer Thoctar deals 1 damage to target creature or player.

Salvat 2011

Alara Reborn

Whenever another creature is put into a graveyard from play, you may put a +1/+1 counter on Deathbringer Thoctar.
Remove a +1/+1 counter from Deathbringer Thoctar: Deathbringer Thoctar deals 1 damage to target creature or player.

If Deathbringer Thoctar and another creature are dealt lethal damage at the same time, Deathbringer Thoctar will be put into a graveyard before it would receive any counters. Its ability will still trigger, but it will do nothing when it resolves.

You may remove any +1/+1 counter from Deathbringer Thoctar to activate its second ability, not just a +1/+1 counter put on it with its first ability.