Night Unicorn sounds good to me.
Boost HD by 2, natural attack damage by a step or two for each natural attack?
An extra 2 Hit Dice is good.
As for the damage, how about matching the 1d8/1d8/2d8 of the original? I like the idea of these being deadly combatants. That's roughly equal to upping the SRD unicorn's attacks two steps apiece.
I'm thinking we could crib some bits off the SRD Nightmare, like its Evil and Extraplanar subtypes:
Large Magical Beast (Evil, Extraplanar) ?
Do we want to alter its stats? If it has a much higher natural weapon damage, maybe it's stronger than a regular Unicorn?
Strength 22 or 24?
Its spell-like abilities look pretty straight forward:
Continuous
silence centered on itself, 3/day
darkness and
invisibility, plus
greater teleport (to dark areas only).
I'd make the
silence a Supernatural power, and am tempted to add couple more supernatural powers - continuous
magic circle against good as a malign reflection of the regular Unicorn's apotropaic powers, plus an at-will
darkness centered on the unicorn. Might as well give it
detect good as an at-will SLA as well, since it's an evil version of a Unicorn.
Finally, I fancy downgrading
greater teleport to
teleport but letting the unicorn use it more often. There's something about the whole "teleport into darkness" notion that makes me think it's less reliable than a Unicorn's teleportation.
So, something like:
Aura of Darkness (Su): A (
dark?) unicorn can generate darkness at will as a free action. This matches a
darkness spell (Caster level 8), except it is centered on the unicorn and moves as the unicorn moves.
Magic Circle Against Good (Su): This ability continuously duplicates the effect of the spell. The
X unicorn cannot suppress this ability.
Spell-Like Abilities
X unicorns can use
detect good or
silence (self only) at will as a free action.
An
X unicorn can use
teleport (caster level 10th) to move to anywhere that lies in darkness [
5/day? at will?]. If the target destination is illuminated, the unicorn teleports to some other dark location that will accommodate it.
Three times per day an
X unicorn can cast
darkness or
invisibility (caster level 10th).