Not too exciting on first glance, but a worthy ally. He often squeaks into my decks as a random singleton, just based off of his 2-for-2 willpower - it's like having Arwen Undómiel in Lore, a sphere that otherwise isn't as good in that area. His ability is awkward to use, but has enough edge cases that it's relevant at some point. Generally it's good for getting threat out of the staging area pre-quest, and you may sometimes want it to trigger certain Dunedain abilities or push an enemy onto a trap. Being an entry effect on a unique card with a nontrivial cost, this will basically just be a small one-time bonus.

Ok, I've recently found a great use for Mablung in my Lothíriel-Éomer deck: ringsdb.com

First, you select your characters for questing and this happens simultaneously: quest with Éomer and Lothíriel and then trigger three responses: first put Mablung into play via Lothíriel. Then trigger Mablung's response that moves an enemy that was engaged with you to the staging area. Finally, trigger Éomer's questing response. If you manage to play a few weapons on him, he will swing for 6-7. And +5 engagement cost often means that the enemy won't engage you back this round.

With Visionary Leadership and Steward of Gondor, these three characters will quest for 9, which definitely isn't bad.