[Endoperez]

I'm not sure if I read the battle correctly, but to me it seems like:

1) your undead chaff attacks the Tartarian, and is killed, perhaps after your mages rout.
2) Your Catoblepas attack the Tartarian, and are killed.
3) Your golem, which has been casting Drain Life up until now, and the enemy Tartarian finally meet each other. Your golem has attack 13, the Tartarian has defence 20.
4) They face off, your Golem has much more hp. I'm not sure if you had the battle paused while you showed the hp total of the Tartarian multiple times, but it seems your golem managed to keep the Tartarian's regen almost under check, until
5) Your Golem died. The Tartarian went on to kill the remaining longdead.

Is this somewhat correct? The important phase is the longdead and tartarian left after your golem died. If there was still one of your longdead (mindless) on the battlefield, and the auto-rout started, shouldn't it be dead as well? If it was the auto-kill, everyone (including the Tartarian) would have died.

I don't remember how mindlessness and routing works, but to me at least it seems like something else might have happened there. Have you generated a log and checked what it says?