Value of legendary and Lair actions on a CR?

[Albonor]

Hi all,

I am running a campaign (who here isn't?) and I was planning to end it around level 14 for all 3 players (fighter, assassin/revised ranger and fiend warlock). The "second to last" boss would be the undead adult red dragon guarding the ruins of the devastated valley where story takes place, fighting in the streets, in the buildings, etc.

However, for real-life purposes, the players probably won't have time to get further than level 11. I plan to fix this partially by finding a set of six manuals of ability raising (Str, Dex, etc.) on top of strong magic items (frostbrand, staff of fire for the resist, etc.). That still leaves them REALLY behind in the DPR race, even if the assassin scores three criticals on the opening volley.

Now, the undead type is a homebrewed one I call the Revenant (someone sue me). Upon dying, the victim (yes, the dragon was killed recently) has its soul replaced by a shard of a Demon Lord/God sleeping under the world. That makes them get up and go on about their business, without showing any initiative anymore other than general reaction (chasing the intruders, murdering any innocent they see, etc.). It raises Constitution to 30 and gives a vulnerability to Radiant and...Criticals. That's right, when you score a crit you can really mess those guys up. Otherwise, they're tough.

Since it lost a lot of his "mind" with the transformation, can I remove the Legendary status from the dragon and hope this lowers the CR to something they can beat or is my math off?

Fighter (Monster Hunter): Sword and board
Warlock (Fiend, Book): Fireballing entousiast
Assassin: Archer.

[Addaran]

I don't think there's a clear method to calculate the CR change from lair action. It would depend on the base creature and what his lair actions are.
From PotA, there's 4 bosses with lair actions, that changes their CR from:
7 to 9
8 to 12
9 to 12
9 to 13

[Malifice]

To eyeball it, its around 3 CR points.

Why not simply go with a CR 17 dragon (and retain the legendary actions and saves)?

An Adult green is CR 15.

Give it the undead type.
Change damage resistance: Cold; bludgeoning, piercing, and slashing from nonmagical attacks.
Change damage immunities: Poison, Necrotic
Change condition immunities: charmed, exhaustion, frightened, paralyzed, poisoned, stunned
Add: Vulnerable: Damage from critical hits, radiant
Add 2 more HD and up its HP to 330 HP (for Con 30).
Change its breath weapon damage (and extra bite damage) to Necrotic.
Increase CR to 17.
Increase proficiencey bonus by +1 (all saves, skills, perception, DCs and attacks) for the increase in CR
Increase AC to 20
Increase damage dice by 1 step (breath deals 16d8 instead of 16d6, bite is 2d12+6 (and 2d8 necrotic) instead of 2d10+6 (and 2d6 poison)

[SharkForce]

most likely, the CR value of lair actions is based on how much extra damage it allows them to deal per round most of the time. (it could also be based on how much it increases their hit points or AC by, but lair actions that make the target harder to kill seem to be more rare ime).