What's a "Khorne Level"?
Just a shortcut for "skull amount collected" to make formatting a little easier on me. Its not an actual in-game name, I made it up!
You wouldnt want to read "When 1 skull collected XYZ" over and over right?
Now, lets get into the data shall we?

Skulls give you the "Khorne Level" KL for short. One skull = one level.
You can have a KL up to 5.
If 5 or more skulls are collected, the mission will have "Send in the next wave!" enabled permanently.

Levels completed while the challenge is active does not count for deed progression (sadly).
The Skulls appear in normal campaign missions. And they will also appear in Twitch Mode and Weekly Challenges. But not in Chaos Wastes, Weaves or Deeds will have skulls in them.

You will not find pictures of buff skulls in here.
Each KL gives one buff and you keep the buffs from the previous levels.

The buffs per level are the following:
KL 1 = 15% Power Level
KL 2 = 12% Attack Speed
KL 3 = 20% Crit Chance
KL 4 = 20% Movement Speed
KL 5 = 25% Cooldown Regeneration (this works on OE ult!)

The higher your KL the lower the buff duration will be. Making it more difficult to keep the buffs.
All buffs share the same duration.

KL 1 stays active for 90 seconds before decaying.
KL 2 for 75 seconds.
KL 3 for 60 seconds.
KL 4 for 45 seconds.
KL 5 for 30 seconds.

(This part is just for users wanting to know how it was before.)
Before Hotfix 5.5.2 (May 30th, 2024):
KL 1 stayed active for 60 seconds before decaying.
KL 2 for 50 seconds.
KL 3 for 40 seconds.
KL 4 for 30 seconds.
KL 5 for 20 seconds.

What happens when your buff runs out? Well you start bleedin' you lucky bleeder!
And here is how much damage you take!

KL 1, you take 1% of your max HP in damage each tick (19 ticks total).
KL 2, you take 2% max HP per tick.
KL 3, 3% max HP per tick.
KL 4, 4% max HP per tick.
KL 5, 5% max HP per tick.

• The damage CANNOT down the player.
  
• The tickrate is roughly 1 second and the debuff lasts for a total of 20 seconds. Why is it only 19 ticks then? That I cannot tell you. But what I can tell you is that its consistently 19 ticks.
  
• Damage below 1 is not rounded up. (if you have 150 HP, you take 1,5 damage a tick at KL 1!)
  
• Damage taken by this triggers things like "Ablative Armor" (so you lose stacks!), be aware of playing Unchained!
  
• Damage can be reduced by invulnerbility or damage reduction. (the property on necklaces "Damage reduction vs Area Damage" works on this too).
  
• Effects like the grim curse reducing your max HP will not reduce the damage taken by the decay bleed!
  
• You can cancel this effect by collecting another skull. Even if you have the KL 5 decay going on, picking up a skull will result you going back to the KL 1 buff. Which means you will take less damage if that runs out!

You simply do what the Khorne wants, kill and shed blood!
Any kill (except killing your teammates) will reset the duration of the buffs. Be it a boss, an elite, special or even a lil critter rat.

You can also refresh the timer by collecting another skull (which isnt that good if you try to keep a low KL as it will also increase your KL).

And in Chaos Wastes there are boons specific to this event that reset the duration of the buffs, how those boons work is written on the boons themselves!

The rest of the section is just for those who want to know how it worked in 2023 Skulls event. It doesnt work like this anymore in the 2024 Skulls event (and onward.)
Lets explain it in some simple terms. You need 5 tokens (which you gain by, you guessed it, killing enemies) to refresh the duration of your buffs.
Easy enough, now lets get into the details!

• Each KL needs the same amount and no changes to the amount of tokens enemies give.
  
• Elites and specials only include enemies of that type if not mentioned by name in any other section!
  
• "Refresh" simply means you will always refresh your buffs if you kill the enemies in that section.

no token = critters (rat+pig)/loot rat (lol)
1 token = slaverat/clanrat/fanatic/ungor/archer
2 tokens = clanrat+shield/marauder/gor
3 tokens = marauder+shield
1 refresh = Blightstormer/Elites
1 refresh + 2 tokens = Bosses/Chaos Warrior
1 refresh + 3 tokens = Specials/Stormvermin+Shield/Bestigor

Btw. for anyone who isnt aware. The Beastmen Standard Bearer is considered a special despite acting more like an elite.

You can also collect skulls to refresh the timer.
But this part is not useful if you try to keep yourself at a lower KL than 5 since that will automatically put you up 1 KL after all.

Since you made it through this skully guide, have a fun fact!

Getting downed will remove all buffs/debuffs of the KL. Meaning you have to recollect skulls to get the buffs back and no worries about your downed health being sapped very quickly.

And collecting a skull after your buffs were removed through being downed, will get you straight back to the KL you were prior! So if you get downed with an active KL 3, you get KL 3 the next time you pick up a skull!

Now that you have all this knowledge, go out there and collect some skulls! Just like Khor- erm, SIGMAR wanted!