Casting the spell itself is a hard MR check (15 not 11)

Internally, horror set uses unit effect 289. It takes two values of this: 1 (lesser infections, gains horror marks and bursts into horrors) and 2 (primary infection, spreads lesser infections)

Upon hosting:

1. The game checks every single unit on the map in turn. If they have no kind of infection, it skips them
2. A unit with any kind of horror seed infection is 20% to gain a horror mark, no MR check offered
3. 10% chance to add a curse
4. If the unit has a lesser infection and two or more horror marks, it is 10% to burst into a random lesser horror (kills the unit, attacks the province as a special monster)
5. If the unit is a primary infection, picks a random unit in the province. The picked unit is skipped if the picked unit IS the primary infected unit or meets another unknown criterion. The picked unit makes a special MR check (no pen or path boosts) of MR + DRN vs 15 + DRN. Loss or tie causes the unit to gain a horror mark. This continues until 100 units have been considered or 5 units have attempted MR checks
6. If the unit is a primary infection, begin another loop of picking random units. The picked unit is skipped if it has any horror marks AND has no horror seed effect AND is not mindless AND is not a pretender AND one other unknown check. Upon passing these checks, it makes another special MR check of MR + DRN vs 11 + DRN. Failure causes it to gain a lesser infection. This continues until 100 units have been considered or 2 units have attempted MR checks
7. It then does something beyond my comprehension