As you'd expect from the Lord of Change, Tzeentch's realm is the most puzzling. You have to make your way to the Impossible Fortress at the center of the map by teleporting between islands.
How to obtain the soul
Use the process of elimination as you map out the portal network and make a note of where each portal leads. Once you have finally teleported your way into the center of the realm, you can challenge Tzeentch for the soul. Rival factions are also moving through portals, so keeping an eye on where they pop out is useful. The challenge comes from not knowing where portals may lead. To find out, you'll need to defeat forces and select the reveal teleport sigil button or move to one of the white outlines that provide a boon of Tzeentch.
Tips for Tzeentch
When fighting Tzeentch, you'll want your army to be evenly spread out, as spells and ranged firepower can destroy your melee units, and flying units like the Doom Knights can demolish ranged units. The best counter is to use your army to focus down larger targets while your fast attacking units should make short work of any stragglers or small groups. It's very time-consuming, but fighting each battle manually can save on troops in the long run.
For more on Total War: Warhammer 3 be sure to check out How to stop Total War: Warhammer 3 from looking blurry or How to claim the Daemon Prince soul in the Chaos Realm of Khorne — Total War: Warhammer 3 here on Pro Game Guides.