They work for the slime god to control balance in the world.
Crimulan: she's the younger sister of the 2 and is chaotic loud and obnoxious. She's obsessed with the terrarian and expects them to spend every second with her. Shes the slime of the crimson. Pissed off at the brain of Cuthulu 
Ebonian: She's the older, smarter and more put together sister. She gets along with the neighbours unlike crimulan. She's quiet, reasonable and mature. Shes the slime of the Corruption.