The Tavern

There is demonspawn mutation that causes spilled blood to erupt in flames. What determines whatever blood is spilled with attack? Do certain weapon types like swords spill more blood (I've got impression that this is the case, but I'm not exactly sure). Or do you just want to cause high damage or attack often?

For melee attacks it's strictly related to damage dealt. Crawl sets a blood amount equal to the damage dealt, then it's doubled if you have any level of ignite blood mutation. Then with an amount/20 chance, blood is applied to the square. If blood is applied then there's a mutlevel in 3 chance to set it on fire for a duration equal to 2 + mutlevel + random2(2* mutlevel) turns.

Blood can't be applied on squares that are lava or water, are a fountain of water (dry and blood fountains are fine), or are an altar of a good god. You also can't bleed in a zin sanctuary, or on a square that's already blood covered without the ignite blood mutation, but those aren't relevant. The cloud can fail to place if there's already a cloud there, you can't see the cell, or the cell is a solid feature (only the first one is relevant to melee attacks).

There's also a lot of other ways to apply blood, more than I can really give a full list of. Ranged weapons get blood amount equal to half the damage done, and so do effects with the BEAM_MMISSILE flavor, which is used for nonelemental stuff. Most earth and conjuration spells use this, for example lcs, iron shot, stone arrow, searing ray, mystic blast, fulminant prism, and battlesphere, but some other things that are monster-specific like harpoon shot and force lance do as well. Barbs damage also has a blood amount of 2 for both player and monsters, regardless of damage.

In conclusion, this mechanic is really complicated and should probably be simpler.