Obatala

Obatalá was a white dragon who Arkhan rode into battle in his assault on Thar Amphala. As an NPC, Obatalá was played by Matthew Mercer.

Background
Arkhan helped raise Obatalá.

Members of Vecna's cult, led by Delilah Briarwood and a death knight, ambushed Arkhan and his companions weeks before Vox Machina's final assault on Vecna's stronghold; they killed his companions, including Vander, and left Arkhan for dead.

During Vox Machina's assault on Thar Amphala, Delilah and Sylas Briarwood appeared riding atop an undead Obatalá. After killing Delilah, the magic that was keeping the dragon in this state dissipated, and the dragon collapsed.

Trivia

 * In our world, Obatala (also known as Orisa Nla; Obatalá in Nigeria and Hispania; Oxalá in Brazil) is an orisha, venerated in Yoruba-derived religions. He is associated with honesty, purpose, purity, peace, the New Year, forgiveness, and resurrection, which is why some associate him with Christ and/or Osiris. As the divinity of created form, "the old man" is the patron saint of artists, called the Divine Sculptor.
 * In Baldur's Gate: Descent into Avernus, Obatalá was alive when Arkhan had Vecna's hand (therefore, after Obatalá's undead form was destroyed in Exandria). Although this happens outside Critical Role canon, it's possible at the very least that Arkhan or one of his allies took the dragon's remains to resurrect him.