Before Thurinder came along, side quests remained in my binder. On a whim I decided to try him out with a side quest package of The Long Defeat, The Road Goes Ever On and various side quests. It was amazing. Also, his set up ability allows you to grab a key side quest that can further a particular strategy like Beorn and Keep Watch. All in all Thurinder is an aweome addition to the Hero Lineup.
Thurindir gets +1 for each side quest in the victory display.
Setup: Search your deck for a side quest and add it to your hand. Shuffle your deck.
Race Across Harad #28. Lore.
Thurindir is a fantastic splash hero - at only 8 threat he's easy to squeeze into any deck where you want access for healing or card draw, starts with 2 and even if you only take the single side quest it thins your deck by one card, gets him up to 3 and you get the benefit of whatever the side quest you chose was, which can be a significant advantage.
Some good choices if you don't have something specific in mind are Gather Information and Scout Ahead - being able to pick up a combo piece or crucial card early can be game changing, and gaining some valuable intel around what's coming from the encounter deck not to mention being able to send something nasty to the victory display is sometimes crucial to surviving to build a board state. Some other decent picks is pairing him with Dúnhere and using Double Back early to ensure you stay under enemy engagement costs, Beorn and Keep Watch as noted below, in a quest focused deck or in multiplayer Rally the West can reap huge dividends getting it nice and early as well. The Storm Comes is a good one for some like the Harad or Outlands archetype, or in a Bond of Friendship deck where the allies are spread across multiple spheres by default.
While you'd never build a deck around him, a Steed of the North or Wingfoot will allow some action advantage to be able to use his 2 as well. He combo's well with Legacy Blade, whether on him or another hero; usually it's guaranteed 1 for no cost even if you only take the single side quest. He also is in sphere and provides the right trait for Dúnedain Pipe to help sort through your deck if you need to as well.
The one downside I will give for Thurindir is that completing side quests early requires a decent amount of early, and his boost is tied to completing them so it can feel counter intuitive sometimes. Running him alongside heroes that with their own action advantage or with even stats so they can quest in the early game but then fight once you have allies on the table or Thurindir sufficiently powered up.
Overall if you want access to and don't need something specific from the hero slot outside of sphere, for 8 threat cost you're getting quite a bit of versatility