Ability scores are ok, and I'd forgotten Int was so high!
I'm not sure I see the justification in splitting these out as Su; the vampire does that to make it a gaze attack (which is a little annoying anyway). I think I'd like to make the charm monster into suggestion perhaps at 3/day and keep dominate person at 1/day.
Just to be clear, you're suggesting:
Spell-Like Abilities: At-will -
read thoughts, 3/day -
hypnotism (DC14),
suggestion (DC16), 1/day -
dominate person (DC18),
mass suggestion (DC19). Caster level 12th, the save DCs are Charisma-based.
That looks reasonable to me.
Shall we move on to its melee abilities?
The original plant has 4-12 tendrils which can club for 1d6 damage or constrict for 1d4 which it can use to transfer prey to its mouth where they take 1d3 spines + 1 acid damage per round. The mouth doesn't seem to do bite damage, but there's precedent for that.
So, 4 tendrils with Improved Grab, Constrict and Swallow Whole?
The tendrils can be severed - copy the SRD Giant Octopus's tentacle-severing?
I think we'd better increase the Swallow Whole damage quite a bit. 1d6 piercing plus 1d6 acid?
It's mobile, but very slow - speed 10 ft?
The original monster's tendrils and mouth are AC5, the trunk AC3 and the head AC1. I'd average it to AC3 and convert that to an AD&D AC of 17-18.
Putting it together, I'm getting something like this:
Helborn
Large Plant
Hit Dice: 8d8+32 (68 hp)
Initiative: +1
Speed: 10 ft. (2 squares)
Armor Class: 17 (-1 size, +1 Dex, +7 natural) touch 10, flat-footed 16
Base Attack/Grapple: +6/+15
Attack: Tendril +11 melee (1d6+5)
Full Attack: 4-12 tendrils +11 melee (1d6+5)
Space/Reach: 10 ft./10 ft.
Special Attacks: Constrict 1d4+5, improved grab, spell-like abilities, swallow whole
Special Qualities: Low-light vision, plant traits, spell resistance CR+7, telepathy 60 ft.
Saves: Fort +10, Ref +3, Will +5
Abilities: Str 20, Dex 13, Con 18, Int 7, Wis 13, Cha 16
Skills: Bluff +6* (+10 with telepathy), Hide +1* (+9 in undergrowth), Listen +5, Spot +5
Feats: Power Attack, Iron Will, Weapon Focus (tendril)
Environment: Warm swamps
Organization: Solitary
Challenge Rating: ?
Treasure: Standard ?
Alignment: Always neutral evil ?
Advancement: 9-11 HD (Large), 12-17 HD (Huge), 18-24 HD (Gargantuan)
Level Adjustment: —
Combat
Helborn attack with their tendrils, which they use to crush opponents before attempting to swallow them. Each plant has one tendril for every 2 Hit Dice it possesses, and can attack with all of them simultaneously with a full attack. A helborn can attack a Small or Man-sized opponent with up to four tendrils, a Tiny opponent with two, and a Diminutive or smaller opponent with only one tendril. Opponents can attack a helborn's tendrils with a sunder attempt as if they were weapons. A helborn's tendrils have 10 hit points each. If a helborn is currently grappling a target with the tendril that is being attacked, it usually uses another limb to make its attack of opportunity against the opponent making the sunder attempt. Severing one of a helborn's tendrils deals 5 points of damage to the creature. A helborn usually withdraws from combat if it loses two tendrils. The creature regrows severed limbs in 1d10+10 days.
Constrict (Ex): A helborn deals 1d4+5 points of damage with a successful grapple check.
Improved Grab (Ex): To use this ability, a helborn must hit an opponent of any size with a tentacle attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can constrict and swallow whole.
Spell-Like Abilities: At-will -
read thoughts, 3/day -
hypnotism (DC14),
suggestion (DC16), 1/day -
dominate person (DC18),
mass suggestion (DC19). Caster level 12th, the save DCs are Charisma-based.
Swallow Whole (Ex): A helborn can try to swallow a grabbed opponent of a smaller size than itself by making a successful grapple check. Once inside, the opponent takes 1d6+5 points of crushing damage per round plus 1d6 points of acid damage. A swallowed creature can cut its way out by dealing 20 points of damage to the plant's interior (AC 13). Once the creature exits, muscular action close the hole; another swallowed opponent must cut its own way out. A Large helborn's interior can hold 2 Medium, 8 Small, 32 Tiny or 128 Diminutive smaller creatures.
Skills: *A hellborn has a +12 racial bonus on Hide checks when amongst undergrowth. *A hellborn has a +4 circumstance bonus on Bluff checks when using its telepathy to communicate.