Custom monster help trying for CR 7

[Spectrulus]

Large Monster (based on Aboleth)
135 HP (18d10+36) AC 15 Speed 40ft.
STR 21 +5 CON 15+2 WIS 10 +0
DEX 15 +2 INT 5-3 CHA 9 -1
Saves: Str +8 Con +6 Wis +4
Skills: Perception +10 Stealth +4
Multi-attack, two claws, one bite
Claws x2
+9, 10ft, 10 (2d4+5)
Bite
+9, 5ft, 8 (1d6+5)
Legendary Actions: 3
1 action; Make a Bite attack
2 actions; Recover 2d8+2 health
3 actions ; Make a claw attack at all opponents in range

The party has 5 members, Elf Gunslinger, Elf Druid of Dreams, Human Arcane Trickster, Human Conjuration Wizard, and Half Orc Blood Hunter Pact of Undying.

The encounter should be it's lair, a decent sized cave with some pillars to slow it down occasionally. Any thoughts?

[DarthPenance]

So a custom monster, based on the Aboleth how? It looks like it but has no similar abilties
You got the right idea with Legendary Actions, since you imply that it will be a single creature against the party, in t2 and beyond it's basically a necessity, but it can only bite and strike, even with legendary actions, seems a bit, lackluster.
Bite attacks usually have an extra effect, whether it be poison, swallowing or bleed effect.
If you want you could also make a new legendary action, probably using 3 actions, to inflict some sort of save, whether it be frighten or something, if you don't want it to be really magical just make it like a roar and the hostile creatures 60 feet from it make a wisdom save or are fightened for 1 minute.
Also what levels are the PCs, if they are 5 level or higher the monster is quite weak, but depending, a cr 7 monster for a party of level 3s could be a bit overkill.
If you want to make it more fun, since it's in it's layer, give it layer actions, as an introduction of the party to them, don't need to make them extremely powerful, a small shake that forces a Str save or fall prone, this things add flavor to the monster.
And I don't know if I misunderstood but you say that the pillars will impede the monsters movement or the party's? If it's the first it's quite stupid on his part, then again, he has 5 intelligence, but animals do know strategy and they have at max 3 intelligence.

[Cwyll]

Not sure what level the PC's are, so here's a range of ideas.

If it's a 'brain' type creature then make the third lair action a mental blast 'wis save or stunned' on targets within 10 feet of the creature OR a direct core attack from it's head (so you can target the ranged peeps a bit more. Blocked by pillars, so you can reward tactical placements. The melee people are already facing the base attacks anyway.

Does it have eggs as well? exude slime that slows enemies that touch it?
How stable is the lair, if explosive spells start getting used?

[WilliamHuggins]

You did manage to make it CR7, its offensive CR is 9 while its defensive CR is 5, but I do have some problems, I basically ignored two of its legendary actions as using the 1 action cost bite is just more effective, trying to overwhelm a single PC with its 6 attacks a round is just better than healing itself or trying to position itself to hit more than 2 people with 3 action aoe.

The way it is now with 4 CR difference between its defensive and offensive CR, the fight will swing too much depending on the d20 rolls, either your PCs are going to kill it in 2 rounds with good initiative and attack rolls, or it is going to start dropping people if it lives 4-5 rounds.

What I would change is, maybe make its 1 cost legendary action a Dash or half speed movement if you think Dash is too much, make its 2 cost legendary action a bite action and add the original bite some rider damage like poison or acid, and make 3 cost legendary action a 15 ft. aoe knockdown action with dex/str save so it can synergize by using it just before its own turn and atacking the creature that was prone with advantage. To make up for the lost damage , unless you buff bite attack enough that it does not make the creature lose damage, increase its HP/AC a little bit, try to make it offensive CR8 and defensive CR6. Actually let me try it;

Large Monster (based on Aboleth)
157 HP (21d10+42) AC 15 Speed 40ft.
STR 21 +5 CON 15+2 WIS 10 +0
DEX 15 +2 INT 5-3 CHA 9 -1
Saves: Str +8 Con +6 Wis +4
Skills: Perception +10 Stealth +4
Multi-attack, two claws, one bite
Claws x2
+8, 10ft, 10 (2d4+5)
Bite
+8, 5ft, 8 (1d6+5) + 5 (1d10)
Legendary Actions: 3
1 action; Take Dash action
2 actions; Make a Bite attack
3 actions ; 15 ft. rad Thrashing attack, DC16 Str/Dex save or prone

[Spectrulus]

Thanks for the replies!

I was aiming for a bruiser rather than a smart critter, and for some reason 5e aboleth is a brute rather than a controller.

The party is all level 5 right now, with an uncommon magic item each.

I appreciate all the ideas, and I want to especially thank WilliamHuggins for the in depth analysis. I'm going to give it an elemental bite damage instead of acid to fit my flavor (keeping it vague in case my players are on here)

I like the revised HP and actions too.

I'll let you guys know how it goes, hoping it's not a tpk, but if it is, I'll know!