When encountered by Vox Machina, Crysa-Thul's form resembled a large, undead fetus.
Before the Stream
In Vox Machina's first adventure, the group was hired by Fendril Vas in Stilben to investigate the seemingly dubious and dangerous new business conglomerate called "the Myriad". Vox Machina eventually entered a chamber housing a planar rift that pulled the group into a room with Crysa-Thul. Fendril was actually a worshipper of the stillborn god and had intended to feed the party to this deity like many previous mercenary groups. Vox Machina destroyed Crysa-Thul and returned to slay Fendril for his treachery.
In the comic version of the story, Chrysa-Thul only made a brief appearance as the party took him down. It seems that Fendril Vas was not present in this storyline, as Vex'ahlia mentioned that they were being paid "a very small amount" by the town of Stilben to take care of it. They fought it in a forest outside of town.
↑Matthew Mercer spelled the name as both "Crysa-Thul" and "Crysa-thul" on his Vox Machina Wiki (see "Fendril Vas" and "Stilben"). The former is used on this wiki because it was Mercer's most frequently used form.