Well, as for his motivation for joining Lucina, he figured that joining someone who was against his main enemy, Validar (and by extension Grima), would be his best bet. The enemy of my enemy and all that. Validar never paid him much attention, since he figured that even if he fought back, it wouldn't matter because Grima would have been resurrected (and was) long before Damien was old enough to fight, and once Grima WAS resurrected, nothing would stop him.
As for surviving, he traveled quickly and fought off anybody who'd try to stop him, namely risen. He's not terrible in a fight, after all, and being raised in Plegia is certainly good for the constitution.