Lore:
As the most advanced Sorcerer King on the path to dragonhood, once you exclude the presumedly defunct Dregoth and the Dragon himself, the Shadow King is well aware that hiding his true power is fundamental in order to maintain the balance of power between the immortals rulers of the city states.
Nibenay fears the other Sorcerer Kings may unite against him, like they did with Dregoth, if they discovered how actually close he is to dragon ascention. As such, he usually fakes greater weakness and vulnerability than his true powers would entice, and surrounds himself with guards, downplaying his own might in order to fuel the perception that he's still far from truly becoming a dragon.
Should he be faced by opponents strong enough to actually threaten his life, Nibenay's strategy would be that of luring them to a secluded, safe area where he can unleash his true powers without anyone witnessing. It goes without saying that should that happen, nobody but him would leave the battleground - he would kill all onlookers, enemies and allies alike.
Tactics:
Nibenay is a careful planner and an incredibly cunning opponent. Should he face opponents powerful enough to threaten his life, he would make sure to lure them into an area that synergizes well with his interaction with shadows and his great mobility.
Once engaged in combat, the Shadow King can prove to be an absolutely ruthless if even unsubtle opponent. Nibenay uses Defiling Burst as often as possible, relying on Ego Storm while waiting for it to recharge and Arcane Defiling and Zone of Shadows for protection. Should his Shadow Tendrils grant him two restrained targets, he lifts off the ground using the shadow tentacles as support, possibly using the shadows close to the roof of the room to stay out of the party's light sources, making the most out of Shadow Armor and hammering them from range.
Once bloodied, Nibenay becomes more brutal and direct. He tries to use Defiling Rampage every turn he can, unless he has to refresh his Shadow Zone, and the Shadow King's Fury boosts his damage potential while allowing him to benefit from Shadow Armor with greater ease.
There's some obvious mistakes: like Nibenay has no skills, no alignment, no languages.
Some of the powers need a little refining:
The Arcane Defiling power needs a trigger, or otherwise another restriction on its use (free actions that deal damage are obviously too good). "New Power" (is that what it's supposed to be called?) also needs a trigger.
Finally, Nibenay suffers from a problem that most MM1 solos suffered from: lack of actions. He'll die quickly because players will get 5 actions to Nibenay's 1 (maybe 2 with a triggered immediate reaction). In other words, give Nibenay more actions. He should be attacking at least 3 times per round, twice by himself and up to 2 other times for various alternate actions - or otherwise, players should be attacking him less often (although that sometimes makes the fights really long).
Good stuff. I like how you took the existing monster and solo'd it.
There's some obvious mistakes: like Nibenay has no skills, no alignment, no languages.
Some of the powers need a little refining:
The Arcane Defiling power needs a trigger, or otherwise another restriction on its use (free actions that deal damage are obviously too good). "New Power" (is that what it's supposed to be called?) also needs a trigger.
Finally, Nibenay suffers from a problem that most MM1 solos suffered from: lack of actions. He'll die quickly because players will get 5 actions to Nibenay's 1 (maybe 2 with a triggered immediate reaction). In other words, give Nibenay more actions. He should be attacking at least 3 times per round, twice by himself and up to 2 other times for various alternate actions - or otherwise, players should be attacking him less often (although that sometimes makes the fights really long).
Arcane Defiling is actually a WotC ability; it's in the Elite version. For some reason, the first time I saved my version he lost all triggers and all recharge values, and I had to redo them. I forgot to redo the WotC ones.
Arcane Defiling is triggered every time Nibenay does damage with Ego Whip or Dragon's Talon, so yes, it's pretty much unlimited.
The "New Power" is "Nibenay's Demand" from the Elite build.
As for action currency, I followed the MMIII model.
MMI solos had few actions and the problem you outlined.
MMII solos were given more actions.
MMIII and similar books seem to have introduced the "I hurt you when you hurt me" mechanic instead.
If you look at him, on an average turn, without action points or recharge actions, he's doing 2 Ego Whips; assuming 5 opponents, 3 of which melee, all hitting him, that means that between 2 Ego Whips, 5 Shadow Tendrils, 2 Arcane Defiling he's dealing 8d12+25d8+148 damage per turn, and weakening 2 targets a turn.
Now roll in Shadow Armor, him being bloodied, and he's doing 8d12+25d8+7d10+148 damage plus 20 ongoing damage, and 2 weakened targets having to go through 5/10 resist all damage, and targets being blinded and him possibly going out of range... his damage output is absolutely insane and players will have a terrible time downing him fast, which is something they NEED to do in order to kill him.
If you look at him, on an average turn, without action points or recharge actions, he's doing 2 Ego Whips; assuming 5 opponents, 3 of which melee, all hitting him, that means that between 2 Ego Whips, 5 Shadow Tendrils, 2 Arcane Defiling he's dealing 8d12+25d8+148 damage per turn, and weakening 2 targets a turn.
Now roll in Shadow Armor, him being bloodied, and he's doing 8d12+25d8+7d10+148 damage plus 20 ongoing damage, and 2 weakened targets having to go through 5/10 resist all damage, and targets being blinded and him possibly going out of range... his damage output is absolutely insane and players will have a terrible time downing him fast, which is something they NEED to do in order to kill him.
I'm really not all that good with the rules, but according to the PHB, creatures get only one immediate action per turn - or is that just for PCs? So Nibenay would only be able to use Shadow Tendrils or Shadow Flight or Nibenay's Demand once - and then have to wait until the start of his next turn to use it again.
...unless you want to make it so Nibenay has extra/unlimited immediate actions...?