Combat allies is a system in Nazjatar that is one of the ways that characters can level out their powers.
In this system players can take out one of 3 allies. Combat allies gain experience by completing world quests, killing rare spawns or doing their own daily quests that unlock whenever you take one of them out with you. As a combat ally in leveled out they will gain new abilities and eventually unlock other cosmetic rewards. Each combat ally has 30 levels.
The combat allies are part of the Waveblade Ankoan for the Alliance and part of the Unshackled for the Horde.