Fallen: degraded or immoral
Forsaken: deserted; abandoned
Foul: grossly offensive to the senses; disgustingly loathsome
To make normal creatures more vile and dangerous, just simply add adjective fallen, forsaken or foul in it. These creatures are more dangerous than normally would. You can add some HD or other powers to this type of creatures to make them different. But remember, if every creature characters encounter are fallen, forsaken or foul they become normal. Fallen, forsaken or foul creatures should be beyond normal and really nasty. Here are few examples to use with your fantasy game. Be free to create your own fallen, forsaken and foul variations of normal monsters.
The villagers were in desperate need in their poorness what has lasted over an decade so they decided to make a pack with a demon. Demon gave them their normal living conditions back in the exchange of worship. Human sacrifices, eldrich rites and other horrible customs are practiced in the name of the demon. Usually villagers try to use cunning to get travelers sacrificed to their demon lord.
When an angel looses his faith he is cast away from good and thus will be fallen. He might be alone, try to become a demon to be worshipped or might join legions of evil. Fallen angel replaces divine powers with evil powers equal - or worse.
Paladin committed a crime against his faith so bad he was cast away by his god. But he was not left again as evil god saw a possibility. Forsaken by good became cherished by evil. The paladin's armor became one with him growing chaotic forms like spikes and faces of tortured spirits on. Now the paladin is one with his armor and his quest is to do what ever his new lord needs.
HD+2, AC+5, Turn Undead replaced with similar power Turn Good. Causes fear.
Forsaken Orc Tribe Of Bloodbrothers
Bloodbrothers is a warrior tribe of orcs. They are really strict in their customs and organized fierce warriors. In their last big battle some didn't perform as expected thus were forsaken from the tribe. Worse destiny than death for Bloodbrothers. Forsaken bloodbrothers became their own pack to survive. The tribe was everything they knew so they had to learn ways to survive without their tribe. They are more aggressive, fierce and strong than other orcs.
+2 HD, +1 AB and AC. Special power: Rage of the forsaken: When rolling results 19 or 20 with d20 in to-hit gain additional attack. When HP reach 0 or below regain 2d6 HP.
Foul Skeletons Of Necromantic Death Worshippers
Necromantic Death Worhsippers are a cult who worship god of death by mass sacrifices. Their guardians are skeletons made of the remainings of their victims. There are still parts of flesh and other tissue hanging on the bones.
Cause Fear. -1 AB to attackers.
Sometimes dragons when age start to die physically and become half rotten. These dragons are insane for their physical change. But because there is so much dead tissue they are harder to wound.
All damage taken is halved. AC +4.