So Brimstone is actually the leader of the Immolators, for now. Volcano is the supreme leader of course but all the elementals live a temporary existence via their avatars. Brimstone is and was Volcano’s captain of the guard so to speak.
He is a host and he can spit out fire spirits throughout the game. He is probably the most powerful host as fire spirits are the most powerful symbiotes.
Unusually the immolators are split into sub types. There are regular immolators in bot bodies and the entombed.
Uniquely immolators are the only elementals outside of the titans that can encode bot bodies. This means that the regular immolators have taken over a bot code in the ether and have downloaded via a singularity event.
But thats not the only way an immolator can become a bot. When a bot first dies, as the code leaves the body, there are a few seconds in which an immolator can possess the carcass. If this happens the immolator becomes entombed within that carcass. In many cases even modifying it with new powers etc. The most well known of these are thousands of years old and come from the great elemental wars. When the Beastlords attacked a weakened Volcano and destroyed her. Many Beastlords died in this battle and their powerful carcasses were possessed by immolators.