Follow-up to @davidnoonan: a three-stages #dndnext Lich boss!

I was thrilled by Dave Noonan's idea of making a D&D Next Lich into a three-stages boss, like AngryDM did some time ago for 4e Solo monsters.

Since I'm a tinkerer at heart, I instantly opened up the Bestiary and instantly jotted down my version of the undead TPK-causing calamity that is a true "final boss" Lich...

Following Noonan's ideas, I made the first stage a crazy "bombastic" one, the second a sneaky one, to which I coupled the nasty area effects, and the third stage is the desperate one, to which I coupled the last resort high level area spell and a funny physical change inspired by a related classic D&D monster...

Stage 1

The Lich here is an arcane fuel tank constantly exploding and never ending its fuel. Or something like that.
Since it won't last for very long, I said why limiting the spells? If players don't hit this thing hard enough, their fault: the Lich will continue raining Fireballs on them. Luckily, it's just 30 HPs of glory for the Lich.
I also added some customizations of the sidebar and some exceptional rules to make up for lack of actions etc. The strategy of the lich here is to not let the enemies approach it, using its (changed) cantrip for that and hoping its Frightening Gaze works too. In the mean time, just rain spells on the enemy.
Here it is:

Hit Points: 30
____________________

Spellcasting: The lich casts one of the following spells: at
will— frost ray; fireball, lightning bolt, magic missile.
It can cast frost ray as a reaction when an enemy moves within 15 ft
and casts magic missile as a reaction when damaged at range.
The DC to resist its spells is 17 (+2 bonus)

Actions and reactions: The lich has 2 reactions. When stunned, it can't use
reactions, but it can still use actions.

Customizations: Frightening Gaze, Turn Resistance (consider total HP)

_____________________

Stage 2

So the adventurers should get more or less 2 rounds of the Stage 1 madness, but they're up for a few surprises with Stage 2, which I'd call the "grind stage". Here the Lich understands that the adventurers are full of resources, so it aims at depleting them with a longer battle, using invisibility and cheating a bit with it, since its spells don't break it. He still wants to break invisibility at least once or twice though, since it now has Life Drain with its touch attacks, and can have Advantage on both attacks when invisible, as having the Ambush feat (the second attack wouldn't normally have Advantage since invisibility is broken, but we want it to be extra sneaky...). Basically, it aims at doing two "cycles" of invisibility+cloudkill+firewall, then cold aura and frost ray on everyone and then multiattack ambush.


Hit Points: 40
____________________

Stage-Start Trigger: The lich casts Invisibility and Cloudkill as no-actions,
without expending its uses of the spells.

Spellcasting: The lich casts one of the following spells: at
will— frost ray; 2/day— firewall; 1/day— cloudkill, invisibility.
It can cast spells without breaking invisibility.
It can cast firewall as a reaction when an enemy would move out
of the cloudkill area.
It counts as having the Ambush feat when attacking from invisibility.
The DC to resist its spells is 15

Customizations: Cold Aura, Life Drain, Turn Resistance (consider total HP)

_____________________

Stage 3

The second stage should last a bit more than the first, possibly a lot more if played right. So the third stage needs to be short: it is going to be the really desperate fight for undeath-prolonging, with a bit of madness in it. The Lich has taken many hits and eventually its body is destroyed... Leaving only its skull floating, like a Demilich!! In this state, the lich tries to flee, but it's also furious, so it tries to attack while leaving (yes, touch attacks still work, maybe the claws are floating alongside the skull too!). It begins this stage by casting a (low DC) Ice Storm, and then has access to Shocking Grasp, Disguise Self and Darkness. I see it using Darkness as a squid using its black ink cloud while fleeing, and Disguise Self when for some reason it believes the bluff may work..! Note that Turn Resistance is out, so the best thing to do here is Turn Undead!


Hit Points: 15

Speed: 50 ft. (Opportunity Attacks are made with Disadvantage)
____________________

Stage-Start Trigger: The lich casts Ice Storm. It is immune to its effects.

Spellcasting: The lich casts one of the following spells: at
will— shocking grasp, disguise self, darkness.
It can cast shocking grasp as if it was a Swift spell.
The DC to resist its spells is 12

Multiattack: the lich makes two touch attacks at any point during its movement.

Customizations: Nothing: no turn resistance!

_____________________


That's all folks! This is my crazy idea of a climatic and dramatic lich battle in three stages. From the experiences I had and those read from other players, this shouldn't be much more difficult than its suggested original level, perhaps just 8th level instead of 7th. The difficulty in D&D Next combat is given much more by the number of enemies than the strength of a single enemy. But then again, no monsters such as this were ever tested, so it could still prove deadly.
Tell you what, test it and see it for yourselves! :)