Slaad

When a red slaad claws a humanoid creature, it can inject an egg from a gland under one of its claws. The egg works its way into its host and gestates, eventually forming a slaad tadpole. Such a tadpole then eats its way out of the host’s body, feeds on the host’s remains, and then seeks any other fresh meat it can find. The tadpole transforms into a fully grown blue slaad — or green slaad if the host had the ability to cast 3rd level spells or higher — within 2d12 hours.

The bone hooks that protrude from the back of a blue slaad’s hands inflict a terrible transformative disease on humanoids wounded by them. This infection, called chaos phage, transforms its victim into a fully grown red slaad — or green slaad if the host was a spellcaster able to cast 3rd level spells or higher.

Green slaadi are surprisingly intelligent and possess innate spellcasting ability. A green slaad can change its shape to appear as a humanoid. If it was born of a humanoid host, the slaad usually adopts its host’s form. At some unpredictable point in its existence, a green slaad unlocks the means to magically, instantly, and permanently transform itself into a gray slaad. Unlocking this knowledge can take years, even decades.

Outside of Limbo, gray slaadi act as living extensions of the will of their masters, the death slaadi. A gray slaad journeys to the Material Plane on errands of doom, often taking humanoid form. A gray slaad learns how to master the use of a greatsword and imbue it with its own innate magic. A gray slaad that eats the entire corpse of a dead death slaad instantly transforms into a death slaad.
(You have yet to find a text on Death Slaadi)

Infernology

The Spawning Stone. Long ago, Primus, overlord of the modrons, created a gigantic, geometrically complex stone imbued with the power of law. He then cast it adrift in Limbo, believing that the stone would bring order to the chaos of that plane and halt the spread of chaos to other planes. As the stone’s power grew, it became possible for creatures with ordered minds, such as modrons and githzerai, to create enclaves in Limbo. However, Primus’s creation had an unforeseen side effect: the chaotic energy absorbed by the stone spawned the horrors that came to be known as slaadi. Sages refer to Primus’s massive creation as the Spawning Stone for this reason.
The slaadi wiped out every last modron enclave in Limbo. As creatures of utter chaos, slaadi loathe modrons and attack them on sight. Nonetheless, Primus stands by his creation and either doesn’t perceive the slaadi as threats or chooses to ignore them.
Birth and Transformation. Slaadi have horrific cycles of reproduction. Slaadi reproduce either by implanting humanoid hosts with eggs or by infecting them with a transformative disease called chaos phage. Each color of slaad reproduces or transforms in a different way, with red slaadi spawning blue and green slaadi, and blue slaadi spawning red and green. Each green slaad undergoes a lifelong cycle of transformation into the more powerful gray and death slaadi. With each transformation, the slaad retains its memories.
Shapechangers. Some slaadi can transform into the humanoid creatures from which they were originally spawned. These slaadi return to the Material Plane to sow discord in the guise of their former selves.