Three-headed Goblin god of death.

“The moment you gaze upon him, he requires a sacrifice. If he approves your offering, he will grant you a favor: either to kill one of your enemies or spare the life of a friend.” - Gundur

Death-worshippers of Shintal may bear the mark of their god;

A surprising number of them have the same tattoo, a skull with the number “3” inscribed over its forehead.
…
“That’s the symbol for Shintal, the three-headed Goblin god of death.“ - Pulgor, Festival of Heroes (GS-ADV1)

A surprising number of the patients have the same tattoo on their left shoulder: a skull with the numeral “3” written on it. - Festival of Heroes (GS-ADV1)

At the time of Queen Gimnax's rule, cult worship of Shintal was forbidden;

You interview a few of the death-worshippers, but they’re understandably tight-lipped when it comes to discussing the cult. Queen Gimnax recently outlawed the worship of Shintal. - Festival of Heroes (GS-ADV1)

While exploring the Temple of Ulos, you find Castien kneeling at the statue of Shintal.

“I have prayed,” he explains, “and received a vision. I was not meant to die. The gods sent you to intervene on my behalf.” - Castien - Servants of Azema (GS-ADV2)