Click on Grey Knight vs. Bloodletter Playtest rules above and you will find new rules for using Khorne Daemons against your Grey Knights. They are in playtesting but are working pretty well. Normal terminators struggle against them, but you will be seeing some missions posted for this combo soon.
The Warp Rift idea was a brainwave I had while playing Arkham Horror the other day and adds a sense of urgency to the Grey Knights activities on board the hulk. Rules pack for the campaign coming later today.
A note on Daemonic Ward Saves : Some people have commented that daemons in 40k get a ward save. I have many and varied reasons for not giving daemons ward saves in space hulk. Here they are.
1) I have tested it in Space Hulk, and it was cack. It made the game unbalanced.
2) Do Terminators get their ward save? No. Why should daemons be any different.
3) Personally, I think that since these rules are tested to work with Grey Knights, in a background fluff sense, the bolts fired from their storm bolters would probably be sanctified and pass right through the daemony outer shell.
4) In a regular game of 40k, a grey knight shooting hits on a 3 and wounds a bloodletter on a 4 with 2 shots. The daemon then has a 5+ save. This equates roughly to a grey knight having the following chance of killing a bloodletter for each shot. 2/9. A little less than 1/4 shots. Considering he gets two chances at this a turn, those are pretty good ratios.
Now take Space Hulk. A Terminator on his first shot will kill on a 6. He rolls 2 dice (just like in 40k.) He'll maybe get 3 chances at gunning a Bloodletter down on overwatch, but for the 2nd and 3rd shots he will kill on a 5. His chance at not killing any given bloodletter with one of those 3 shots is 10/27, meaning that he has a 17/27 chance of blowing a Bloodletters brain out. Of course, I'm not taking jamming in to consideration here, but you see my point. The math may be wrong, it's been 12 years since I left high-school, but this is logical probability. In conclusion?
No ward saves.
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