Slaadi are well-known to come in several different varieties which function as castes in slaad society . The more commonly encountered red and blue slaads are the lowest forms, relying on brute force and bulk in combat. Above them sit the green slaads, essentially mutants, who are far less common and who make use of innate arcane powers to survive.
Green slaads that survive for more than a century undergo a curious transformation that remains highly mysterious to non-slaadi. They lose much of the pigment in their skin and a great deal of their bulk, whilst their magical abilities drastically increase in power. These so-called grey slaads are truly fearsome mages, and can prove difficult to deal with even for seasoned adventurers. However, some grey slaads choose to undergo a further, more sinister metamorphosis.
Nothing is known of the ritual that produces a death slaad. We may assume that the participants are willing - perhaps not. Perhaps all slaadi dread to eventually face it. Whatever dark magics are involved, the affected creature is changed utterly, becoming a single-minded entity of chaos and destruction. Death slaads are disciples of murder, pure and simple. They sit at the top of most slaad societies, relying on the (wholly credible) threat of violence to maintain their position.
I really like slaads, but I found this image really hard to work out! I won't bore you with the details but I went through a bunch of iterations. Slaads are just kind of featureless and froggy in most depictions, and it was tricky trying to work out a direction that I wanted to push it in.
The weird exposed gill-things on the shoulders & arms are inspired by the axolotl, bizarre, perpetually-juvenile salamanders that are (sadly) dying out in their natural environment. They have this weird, slightly gross quality to them which I like - like the death slaad ritual somehow provokes this strange evolution in the creature's body.
The "horns" are a cast-off from another idea I had about the death slaad ritual where the ritual involves giving the slaad this strange cordyceps-like fungal parasite which twists it further into this demented killer. I didn't want to make the creature look too "fungal" because I thought it might resemble the Verdant Prince, but I liked the wonky horns so I kept them.
Also I like how death slaads are more keen on melee combat, despite being spellcasters the whole rest of their lives. I like the idea that they got so advanced with magics that they can't even be bothered to do it anymore.