Blood fueds and Revenge

Oops we killed a really vindictive guy...

Mortal table for more mortal folk you kill like nobles or dragons and planar one for beings from other planes like elves or demons or elementals.

So your players meet some other planar being. The mortal form it manifested is destroyed, and its spirit returns home. Once its true native form is complete, the entity will be quite miffy and embarrassed. Maybe higher-ups get involved when they report their failures. But such a sleight by mortal beings messing with the fate of superior beings cannot be just forgotten. Making a new body on the mortal plane may be limited a while, depending on the type. Greater beings find it harder but some specifically recover faster, like night hags.

In my more mythic games, monsters are the bitter venom of the gods and doing their job. Often they have a monster saved for some hero after causing enough suffering. Killing the monsters not at the right time makes the gods angry. Monsters and even bandits might have family. Turns out that bandit was a bastard of a high-ranking lord who feels obligated to kill the adventurers doing their duty. Supernatural parents might spy on their children or feel honour-bound to kill the killers of their spawn (or kill the killers' spawn). 

Beef from the fancy bosses adventurers kill gives DM many fun opportunities. Enemies might not be obvious and act from afar. Perhaps those thugs you kill turn out to be the kin of the first guy you killed, and now the whole clan hate you, and you dont really know whats happening. They might decide their enemy should feel loss and suffer before dying. Perhaps take away the killers loved ones. Even if your poor and powerless just burn their house down or give all their grooming implements to some leppers then return them. Eventually, the tormenter will want to make a declaration and might overplay their hand against the adventurers. So when you kill this boss after a fight you just escalate to the next higher tier of a blood feud. A king or nation might have power used for revenge by trickery, forgery or bribery. 

I like in Viking sorties like Burt Njarl the hero, estimates the legal consequences of attacking guys who hunt him. If he strikes first he could calculate how much money and cattle he would pay if he attacked first. But this is weighed up with, do I let a bunch of guys attack me first so It will cost less to kill them if they start it. Some factions might be paid off or the escalation of the fued limited. The origional angry person might refuse their boss declaring a fued over and may spilt ties or betray the boss. Perhaps the power and drive helps your victims kin gain more power and experience or even adventures.

Imps, sprites, pixies and similar beings can be great for petty revenge and can take a while to discover and kill. 

The higher your level the more blood fueds you should have.
Perhaps every visit to civilisation or your domain should have trouble, from enemies.

"Honestly we had to take over a kingdom because of that guy I killed in a pub fight that led to assassins and fighting a noble clan, and then having to nobble the king"

d12 Planar Revenge Table
1 Try to form a new body and attack as quick as possible

2 Scheme to get a mortal fanatic cultist to avenge them

3 Scheme to get a mortal cult to avenge them

4 Scheme to have cult assassins stalk the enemy

5 Gets an ally to send bad dreams to haunt the enemy

6 Bully a lesser being of the home plane to attack a foe

7 Bully a lesser beings of plane to have a go at wrecking foes life

8 Gets one of its peers to attack for a favour while recovering

9 Gains surveillance while plotting to attack the enemy at a time of weakness

10 Gains surveillance while plotting to ruin the enemy's life (property & loved ones)

11 A higher tier being takes over the matter of vengeance, sending cults & minions

12 A higher-tier being sends a planar hit squad to punish meddling mortals

d12 Petty Mortal Revenge Table
1 Creature has bitter orphans or kin

2 Vindictive best friend who swears vengeance

3 Parent devotes life to causing enemy to suffer (attack foe, allies and property)

4 Kin uses influence to have a war band to kill the killers

5 Kin calls on a spell caster to curse foe harm or damage property

6 Kin put out a wanted poster offering a reward to kill the foe

7 Vindictive feelings at death attract evil spirit into the corpse and using the remnant of the original soul for knowledge of the host body past it can twist into evil

8 Remains used to create undead or flesh construct, and the memories of death make it go off the creator's mission plan. 

9 Kin hires a secret assassin order to kill the foe and they wont quit

10 Kin gives wealth to a temple to punish the foe (priests or divine beings)

11 Kin does dark deeds for unusual power to kill foe

12 Spirit of the dead bound to a location or item. (maybe its favorite magic item or a trinket or artwork or natural features like rocks, a pool or a tree). The spirit will use other spirits or may manifest a body or other non-corporeal undead types. May possess or deceive a mortal to kill an enemy

d12 Cheap Petty Revenge
1 Poisoning

2 Arson, burn property or inn they stay at

3 Rob cash of foes' loved one or pet

4 Assault foes' loved one or pet

5 Steal or sabotage foes' property

6 Pay gang to beat foe

7 Set authorities on the foe with a complaint or a falsehood

8 Spread malicious gossip about the foe

9 Advertise bounty but have no money

10 Hire an assassin, but dont have enough money

11 Pay bandits or humanoids to ransack and pillage property

12 Pay spell weilder to curse the foe or cast a destructive spell on property

d12 Rich Petty Revenge
1 Hire a bandit gang
2 Hire theives guild or gang thugs
3 Have foe declared outlaw or declared a traitor
4 Put out bounty, wanted posters or murder contracts

5 Hire robberknight and warband of scum

6 Hire a mercenary company

7 Have a spy infiltrate the enemy service to find weaknesses

8 Blackmail allies or loved ones of the foe to betray them

9 Bribe allies or loved ones of the foe to betray them

10 Kidnap allies or loved ones of the foe as hostages

11Hire a magician or a cult to call a barrage of supernatural harm and property damage

12 Make a pact with a supernatural being or powerful monster

d12 Weird Cruel Revenge
1 Send mercenary kidnappers to kidnap the foe in a dungeon to be tortured for years by a famous torturer or perhaps in person
2 Send a mercenary force to kill and destroy the home and allies

3 Fake documents that cause a cult to believe they must sacrifice the foe
4 Fake evidence to initiate a blood feud with a vindictive faction of psychos
5 Construct weird assassin order of fanatics obligated to never quit

6 Have foe seduced and place an agent with them, if possible, break their heart and cause them most pain possible. Repeat if possible for extra thrills

7 Hire bards to slander the enemy as a repulsive pervert, heretic, criminal traitors with catchy new songs

8 Send enemies who curse those who kill them tosoften party before real attack

9 Build a dungeon of traps baited with the perfect bait where you can see them die

10 Have a cursed artefact delivered to them. Even better make the side effects bad but worth it. Can then contrive situations they use the item. Cursed divine items might hide their nature but you could research them if blasphemous books 

11 Send an army after the party who will destroy and burn any allies territory

12 Call on a god or entity for supernatural support (sold their soul?)