Cleon
Legend
Good ideas all around. Updated.
How does this look for the tri-beam writeup?
Tri-beams (Su): A beholder pilot can channel its eye rays through the sentinel's trident to produce a magical tri-beam once per round as a free action. These three black beams of negative energy may be targeted at a single creature within 60 feet. A successful hit deals 6d6+3 points of negative energy damage (a successful Will save halves the damage). Additionally, a creature damaged by the tri-beam must succeed on a Fortitude save or fall unconscious for x rounds. The save DCs are equal to 10 + 1/2 sentinel's Hit Dice + Cha modifier of beholder pilot (18 + Cha modifier of beholder pilot for a typical sentinel).
Since undead are powered by negative energy, a tri-ray cures such a creature of a like amount of damage, rather than harming it. A tri-beam follows all the rules for a ray (see Aiming a Spell, page 175 of the Player's Handbook).
I'd prefer it to be a standard action or an attack instead of a free action. The original says it "The Sentinel points the weapon and projects three beams of black energy at its chosen target", not that it can go tri-beaming while skewering opponents with its trident.
The range is also short - the original was 60 yards, so I'd make it either 180 feet or 200 feet depending on whether I want to round it up.
Finally, I'd rather make it force damage rather than negative energy. If it healed Undead you'd think that would be mentioned in the description.
Oh, and there's no mention of reduced damage on a save, so I'd leave the save to just cover the consciousness save, which should last 1d4 round according to the original description.
Put all that together and you get something like:
Tri-beams (Su): A beholder pilot can channel its eye rays through the sentinel's trident to produce a magical tri-beam as a standard action. These three black beams of energy may be targeted at a single creature within 180 feet. A successful hit deals 6d6+3 points of force damage. Additionally, a creature damaged by the tri-beam must succeed on a Fortitude save or fall unconscious for 1d4 rounds. A tri-beam follows all the rules for a ray (see Aiming a Spell, page 175 of the Player's Handbook). The save DC is equal to 10 + 1/2 sentinel's Hit Dice + Cha modifier of the beholder pilot (18 + Cha modifier of beholder pilot for a typical sentinel).