Tharendril

 was one of Vax'ildan's instructors while he was growing up in Syngorn.

Vox Machina - Kith & Kin
Around 798 PD, Tharendril was assigned as a combat tutor to Vax'ildan by his father Syldor Vessar. Tharendril apparently only accepted to do so because he owed Syldor a favour. He was present when High Warden Tyrelda visited Syldor's home to inspect the twins' progress.