Demongo, also known as The Soul Collector, was one of Aku's strongest minions and was an antagonist from Samurai Jack. He was sent after Jack after he managed to stay alive after Aku's constant attempts on his life.
Little is known about Demongo's origins or past. The creature may actually be some creation of Aku, due to how he held a skull that held his essence after he was ultimately defeated, and does have some minor semblance to Aku, dark seemingly malleable body and a fire motif as well. He was likely Aku's weapon against heroes, as he had an entire army of skilled, but defeated, heroes against his target until they were slain, allowing him to consume their soul and essence and further increase his power.
Demongo manages to survive a little longer than other villains (besides Aku) in the series. After Jack frees the souls entrapped within him they are resurrected as flesh and blood warriors, rendering him weak and powerless. The only power that might remain is his teleportation. When this happened, he was confronted by the now-living warriors and was quickly overwhelmed and killed. His essence was then absorbed by Aku into a glass skull. Demongo (as an essence) begged Aku for forgiveness, but Aku refused and crushed his prison. (Episode XXIII: Jack versus Demongo, The Soul Collector)
Demongo returns in Episode XCVII having somehow survived when he enters the Da Samurai's bar while he explains his story to Ashi. He states that he is searching for the essence of powerful warriors to steal, but comments that there were none at the bar and leaves. (Episode XCVII)