What are the effects of challenging the boss with multiple skulls active?

When you reach the boss chamber, the sigil on the boss's gravestone is surrounded three interlinked skulls.

Defeating enemies on the level darkens these skulls one by one.

Does the number of lit up skulls have any effect on the boss fight?

Solution 1:

More skulls = more small enemies to deal with during the boss battle

Couldn't find anything on the wiki or any fan sites, so I decided to just test it out myself to see what stands out.

Boss with 0 skulls

With all the skulls off, only two additional enemies spawn with the boss.

Boss with 3 skulls

With every skull still on, 6 additional enemies spawn with the boss.

The number of skulls doesn't seem to affect health or moveset, so it seems more skulls just means having a busier boss battle. Additionally, it seems the extra enemies spawned by having skulls lit up are always just Ghouls, so if your current loadout isn't ready for crowd control, you might want to clear out more of your current floor first.