Yeenoghu

"Yeenoghu? Yeenoghu!"
 - That one Gnoll Lydia decided to adopt

We know next to nothing about Yeenoghu, save that he's a gnoll, was born on Ekellos, and was an old ally of Atrox. According to a random gnoll Lydia decided to adopt, he is also the god and creator of all gnolls.
Despite this small bit of information, there are some infrences we can draw. He must be very old to have been an ally of Atrox - at least 800 years, almost certainly more. For Yash'ailaran to have not heard of him, he was likely either not particularly influential, or was kept secret. The fact that he levelled the Fey fortress even after the death of Atrox suggests that he remains something of a loyalist. 
Finally, we know that Yeenoghu's current goal is to revive a corrupted version of Atrox by locating a piece which the almighty hid of himself in case of his death. This would certainly mean death on a planetary scale.