I also use custom monsters, I think some of the more memorable mechanics I've used were from my more recent solo designs

From an Angel of Mercy (leader type monster, mostly summoned more enemies, healed and enabled them). Level 20 monster
Bond of Mercy
At-Will ✦ Standard Action ✦ Healing
Close Burst 10 ✦ One bloodied enemy
Effect: The target may choose to spend any number of healing surges. For each surge the target spends, they suffer a -[[5]] penalty to Will against this attack
Attack: +23 vs Will
Hit: The target cannot deal damage until the end of the encounter. This effect ends if the target takes damage.
The fight did contain ways for the players to damage themselves (running into rips in reality that would slap them around for multiple surge values of damage) or they could get attacked by their allies.

For the Angel of Corruption that was the centerpiece of the following fight (the same angel revealing it had been corrupted by the Far Realm), it instead had this option
Bond of Corruption
At-Will ✦ No Action (limit once per round)
Close Burst 10 ✦ One bloodied enemy
Effect: The target may choose to spend any number of healing surges but regain no hit points. For each surge spent, the target gains THP equal to its surge value. For each surge the target spends, they suffer a -[[2]] penalty to Will against this attack
Attack: +23 vs Will
Hit: The target is dominated (save ends). For every surge that was spent, the target suffers a -[[1]] penalty on saves against this effect, and gains a +[[2]] bonus to attack and damage rolls until the dominate ends.
Basically, they both offered you a powerful heal, which was important because this fight had a roaming angelic blast that would hit a bust 5 every round for irreducible max HP damage. But they also both came with a drawback.

The other fun one I had was when I decided to make a monster based on the Endbringer Leviathan from the web serial Worm. Leviathan's iconic ability is that he has a water double that essentially repeats his attacks. He also leaves behind walls of water wherever he moves. So I gave him these two traits
Afterimage
When Leviathan moves, it leaves behind a trail of water. This trail is made entirely of water, and requires Athletics checks to swim through as stormy water (DC 20). These walls of water provide Leviathan with Total Cover against attacks that pass through them.
Afterimage Attack
At the beginning of Leviathanís turn, the afterimage moves and catches up to it. All creatures in the imageís path take [[floor(@{AL}/2)]] damage and are knocked prone. While moving, the Afterimage will also repeat Leviathanís attacks, targeting the same locations. The afterimage is unaffected by any conditions or penalties on Leviathan.
Together, they gave him a defensive measure (the total cover), an offensive one (repeated turns), and an anti-control effect (since penalties to attack, damage, and actions wouldn't affect the afterimage attack). The repeated attacks were particularly nasty because a lot of his attacks were big blasts, and since you knew the followup was coming, it created areas that the PCs had to avoid standing in.