The Realm of Chaos ruled by the Khorne is the most straightforward realm you can enter in Total War: Warhammer 3. The realm also houses an extremely powerful weapon.
How to obtain the Chainsword
The roar of Ursun opens gates to the chaos realm, one for each of the chaos gods. The one you'll be looking for is Khorne. As you enter Khorne's realm, you'll notice a new battle map, and if you look to each corner of that map, you can find different weapons, including the Chainsword.
Special notes about the Chainsword
All weapons you can find have enabled magic attacks, armor-piercing weapon damage, the Unbreakable attribute, and extra ability. The Chainsword has a passive that increases base weapon damage and armor-piercing weapon damage up to 300% depending on each kill made by the unit. This will be most useful in the hands of your hero. Khorne will take away the weapons that he’s granted if you enter a different Chaos God’s realm, so be careful.
For more on Total War: Warhammer 3 be sure to check out How to fix the no text error in Total War: Warhammer 3 or How to prevent crashes when alt-tabbing in Total War: Warhammer 3 here on Pro Game Guides.