Here's a giant list of different debuffs that a raid/party could be applying (vulnerability from multiple sources applies the stacks faster but don't increase the total number of stacks- not all vuln sources are listed here). There's also a few things missing from this list:
LGS(dino & raid weapons):
Dust -35 PRR / -100 HAMP (5 stacks of -7/-20)
Ash -21 mrr / 60 USP (3 stacks of -7/-20)
Ooze -10 prr/mrr
vulnerability 20% more damage (1% damage, 20% max stacks)
Filigrees:
Shattered device:
4 Pieces: Your Melee and Ranged attacks have a chance to reduce enemy PRR and MRR by 10 for 10 seconds.
5 pieces: Your Melee and Ranged attacks have a chance to reduce enemy Melee Power and Ranged Power by 10 for 10 seconds.
One Against Many:
4 Pieces: Melee attacks have a 30% chance to debuff enemies melee and ranged power by 20 for 10 seconds.
Epic destiny:
EA mantle -5 prr/mrr
EA shadows upon you -5 prr/mrr
SD darkness -9 prr/mrr (-3 prr/mrr stacks 3 times)
SD shadowloss -5 prr/mrr/spell resist (lasts 5 minutes)
Fatesinger Harmonic Resonance: 10% vulnerability to Sonic damage & -5 AC (stacks up to 3 times)
Classes:
Warlock:
Consume -10 prr/mrr/spell resist (-2 per stack, stacks 5 times)
Devour the Soul -20 PRR and MRR for 6 seconds, 15 second cooldown (doesn't need to land DCs, equivelent to -8 prr/mrr sustatained)
stricken -20 MP/RP/spellpower
stricken vuln stacks (1% damage, 20% max stacks)
taint the blood -4 fort save
Mindwrack: -4 Will saving (AotS archetype)
Monk:
Jade Strike Increases Physical Vulnerability by 10% and reduces fortification by 25%
Alch:
Melt Armor -4 PRR / -4 AC
Caustic Overload -5 PRR
Voltaic Burst -5 MRR
Various sources of fortification reduction:
Assassin's Trick (25%)
Cracking Attack (up to 20%)
Destruction (enchantment) (up to 15%), for example via Deconstructor
Expose Weakness (50%)
Improved Sunder feat (25%)
Insult (10%)
Mark of the Hunted active use (25%)
Shattering Strike (15%)
Shield of Condemnation (up to 50%)
Sunder feat (25%)
Touch of Despair (dark, dark, dark monk finisher) (25%)
Rogue
https://ddowiki.com/page/Mechanic_enhancements
Ooze Flask:
reduces the Armor Class and PRR of your opponent by 2/4/8 for 30 seconds
https://ddowiki.com/page/Ameliorate is another. But it's bugged right now and only works on undead despite it's description to debuff all mobs (does debuff undead that it doesn't insta-kill correctly)