To appease Khorne to approach the Brass Citadel in The Blood God’s Domain in Total War: Warhammer 3 you will need to fight in the Blood God’s Domain and build up 500,000 bloodshed points. This can be done in 2 ways:

1. Fight the armies that are scattered throughout the Blood God’s Domain or
2. Fight any armies in the Blood God’s Domain from rival factions.

I found that it took fighting about 5-6 of the armies until I had built enough bloodshed. You will then be able to approach the Brass Citadel to complete the quest battle. I found the best way to be able to keep fighting was to encamp and slowly move towards the various armies in the Blood God’s Domain who will come and attack you if you are close enough. The encamp stand allows you to replenish and I also usually picked the replenish option after winning fights.