A damage projectile is enchanced with debuff qualities(via optional upgrade): Typically there is a choice to enhance damage, fire rate or debuff.
Typical debuffs(debuffs which stack are applied at small quantity e.g. -1/-2 for 10sec, while non-stackable debuffs work for longer period at percent value e.g. -30% for 40sec) :
1.Reduction of enemy speed or rate of fire.
2.Reduction of enemy regeneration speed.
3.Reduction of enemy light radius/sight range.(e.g. -1 to sight radius)
4.Reduction of enemy mana/energy/spellforce.
5.Additional elemental/magic damage over time(e.g. poison).
6.Reduction of enemy resistance/defense values(temporary or permanent).