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The Rhaazke are a type of demon from a dimension only accessible from Hell itself, the same one as true hellhounds. Summoning them does not work directly from the mortal plane. Scorn is the only accidental exception due to a chaos effect occurring during the summoning of a succubus.
They are sapient beings and have a matriarchal society.[1] Some of them serve as commanders in Hell's armies.

“The demon in the cage,” Vadrieny said grimly, “is called a Rhaazke.”
“I’m not familiar with that species,” Trissiny said, frowning. “Do they resemble Vanislaads?”
“About seven feet tall,” Vadrieny said, “very muscular, mottled skin. Slitted eyes. Claws, horns, feet like mine…no wings, but they do have spaded tails. Physically quite powerful, and gifted magically. I’m not surprised you haven’t heard of them, Trissiny; I don’t know much surface-level demonology, but it would be very hard for one to get to the mortal plane ordinarily.”

Rhaazke are culturally sort of like drow: matriarchal and militaristic. They are also loyal to Elilial, and emotionally stable, like hethelaxi without the berserking. In fact, those two things are related. It was their pocket dimension that Elilial launched her first campaign against Scyllith from. She bought their loyalty and keeps it by altering them so they don’t lose mental stability to infernal effects. These creatures are dangerous.”