Is my homebrew lower-powered Lich's CR calculated correctly?

Question

I created a "Lesser Lich" so my party would be able to fight something interesting in the story I have. In my game, a lich undergoes development changes the first time it "ascends(descends) to lichdom." It first becomes a protolich (lowest level) for 1d10 days. This creature is mostly feral, acting more instinctively. It also gains no lich lair bonus. It then becomes a "Lesser Lich" for another 1d20 days or until it makes an appropriate number of sacrifices to its phylactery. This made up stuff doesn't apply to a lich that was already a full lich.

Here is the stat block I came up with:

My question is, is my "Lesser Lich" really a CR 10 (12 in lair)? I used the DMG to calculate, but it was a bit confusing.

My Calculations

Defensive CR 4 (hp 2, AC 10; move 2 up 2, 4)

Offensive CR 16 (this I really guessed on... dmg/round? 50-100?)

average the two: 10

My damage per round, I had to guess. If the lich casts, "disintegrate" it can do a max damage of 100. That's is one big move. If it doesn't... I have no idea how much damage it does in a round.

Sources / Credits

• DMG page 202-203 for Lich base stats
• http://valloric.github.io/statblock5e/ for the statblock template.

addition / edit:

Based on the feedback, I lowered the level of my "lesser lich" to an 8th level caster. This cuts 5th and 6th level spells and lowers the HP to ~74. I also lowered the AC to 15, reduced legendary actions to 2 / turn, and recalculated damage. Based of this, the average damage I got was ~ 57/ round (Blight hitting 2 creatures for average damage, and 2 ray of frosts). All this gives me a def CR of 8(?) and Off Cr of 10(?). CR is 9 out of lair and 12 in lair(?):

Based on my own play test vs a group of 4, the second block is probably a CR 6-7.

Answer 1

CR10 is low for this creature. Here's how I computed it:

Assuming a target CR of 10, offensively you have:

• 111+ damage/round (3x Ray of Frost plus Cloudkill or Disintegrate)
• +9 attack bonus
• DC17 save

Damage is CR18, but attack and save are low for that and drop it to CR17.

On defense you have:

• 86 HP
• +20 effective HP for legendary resistance 1/day
• x 1.25 for significant resistances
• AC 17
• Three saving throw bonuses (effective AC +2)
• Access to globe of invulnerability and counterspell

Adjusted HP is CR5, AC is 4 points higher than suggested for that CR to increase defensive DC to 7. The defensive spells are worth (I think) at least two CRs if not more. Bear in mind that with an Intelligence of 20 the lich will probably be prepared for an encounter with the globe and mirror image up. This means it ignores spells (including counterspell) of level 5 or lower, and it can use its own counterspell to shut down one dispel magic attempt per turn.

So conservatively it's offensive CR17 and defensive CR9 or more, which average to CR13 without the lair bonuses. CR10 seems like a significant underestimate.

The lore around the creature is fine, however.

Answer 2

Let's assume that the target CR is around CR 10, for calculations related to legendary resistance and damage resistances.

For defensive CR (6):

• 74 HP
• Damage resistance (1.50 multiplier)
• Legendary resistance (20 effective hp)
• 3 saving throws (+2 to effective AC)
• Effective hp: (74*1.5) + 20 = 131
• Effective AC (15+2) = 17
• CR for hitpoints: 5, defensive CR after AC: 6

Keeping in mind the legendary action that "transfer damage" on the lair action, we have that the lich double it effective hit points on his lair (for a total of 242) (Reference: Damage transfer on monster features, DMG pg 280). So while in his lair his defensive CR is 12

For offensive CR (12):

• 2 legendary actions with ray of frost (18 avg dmg / round)
• Level 4 fireball (31.5 avg damage * 2 = 63). AoE are expected to hit 2 targets.
• Level 3 fireball (28 avg damage * 2 = 56). AoE are expected to hit 2 targets.
• Most damage comes from ST, so let's use the DC for damage calculations (DC 16)
• Combat is expected to take 3 rounds, he is gonna use 2 legendary reactions with ray of frost per round , and cast two level 4 fireballs, plus one level 3 fireball for a total average of: (((18*3)+(63*2)+56))/3) = 78
• CR for pure damage: 12
• The DC doesn't change the final CR, so final attack CR 12

Again keeping in mind "transfer damage" we should add 1/3 of the monster hit points to average per round damage. (78 + (74/3) = 103) that's attack CR 15 after DC adjustment for lair CR.

With this we have a CR of 9 and CR of 14 with lair actions.