Find the drunkard that repops at bars throughout Alyria.
Common locations:
Spam in and out until he triggers a fight. You can flee and attack him after he aggros and still get the mark. Calm with herbs or spells when he's at agony, while you're still at or near full health. (This may be tough in Tellerium.) If he tells you that you fight like a girl, you're likely not at high enough health.
Note: Multiple people can get the mark off the same drunkard. He won't aggro until he's at full health, so just heal him up after calming and he's good to go for round two.
For showing self-restraint and not killing your aggressor, the Powers have granted thee a Mark. You have been granted the Mark of Temperance! You gain 1000000 experience points! You gain 1000000 experience points! You gain 1000000 experience points! You gain 1000000 experience points! You gain 1000000 experience points! You gain 1000000 experience points! You gain 1000000 experience points! You gain 312,500 experience points! You gain 38 practices!