On this page of the guide to Total War Warhammer 3 you will learn how to get to the Brass Citadel in Khorne Realm.
After entering the Khorne Realm, you will see a new bar above the army. Your only task is to fill it. You will do this by fighting the armies in this world. The more creatures you kill, the more full will the bar become. After completing it, you will be able to fight the daemon prince himself.
Remember, however, that other nations will get the same chance, so if several lords have their bar full at the same time, the one who fights the first match, wins.
You may need the Encamp mode to replenish your losses. However, there is also another, more effective way - artifacts.
There are powerful artifacts in the nooks of the Khorne lands. They generate Khorne corruption as a side effect, but collecting them also allows you to fully heal your units. Therefore, use them when the army suffers from attrition. Remember - the artifacts will disappear when you enter the realm of another god of chaos.
Attack other lords first, who came with the same purpose as you. These fights are more difficult, but your bar will charge faster and you will get rid of your opponent. When your army suffers enough losses or you are just before the final fight, then it is worth using the artifact and healing all soldiers.