Soul of an Ash Ghoul

Once you've reached the rank of Wizard, Skink-in-Tree's-Shade in the Sadrith Mora, Wolverine Hall: Mage's Guild wants you to get an Ash Ghoul soul for him to research (player->addSoulGem ash_ghoul Misc_SoulGem_Grand). He suggests to check the Yakin cave.

Yakin is a cavern northwest of Tel Aruhn, just west of Sadrith Mora on the mainland coast. But you can soul trap any Ash Ghoul from any Six House hideout.