How about
Spell-like Abilities (Sp): Caster level X.
At-will: detect chaos, detect evil, detect good, detect law
?
Well, if we want to reflect the original description, it'd be something like:
Alignment Sight (Sp): As a full-round action, a busen can view the alignment aura power (chaos, evil, good, or law) of every creature, object or spell it can see within a 60 ft. cone. To determine a target's aura power, see the detect chaos, detect evil, detect good, and detect law spells. The busen is never stunned if it detects an overwhelming aura with an alignment opposite to its own. This power requires line-of-sight and is blocked by fog, darkness, and other factors that limit normal sight. It cannot see through cloth, so is useless against creatures wearing garments that conceal their entire body and face, such as hooded cloaks. Busen usually demand intruders disrobe and remove all weapons and jewelery before using Alignment Sight, to prevent the intruders concealing aligned objects and creatures in their clothes.
This is the equivalent of a Xth level spell (caster level Y).
Hmm, it doesn't really sounds like a continuous ability to me, so that Detect Alignment seems better to me. I do think we can cut the 2nd paragraph, as people are supposed to look up the spells. Is this acceptable? 2nd level spell? CL=HD? (Incidentally, I like it, but do we have a good justification for dropping the stunning from powerful auras?)
Detect Alignment (Sp): A busen can examine the alignment auras of every creature, object and spell within a 60 ft. cone. This give the busen the same information as studying the area with the detect chaos, detect evil, detect good, and detect law spells, except it registers all alignments simultaneously and the busen is never stunned if it detects an overwhelming aura of an alignment opposite to its own. This is the equivalent of a Xth level spell (caster level Y).
You mean something like a +12 Sense Motive bonus? Yes, I'd approve as long as we call it out in flavor and/or tactics.
Tentatively updated.
Good point. Well, should we get on with skills and feats before we make the final call about that?
Appropriate skills would seem to be (all at 11 ranks)
Balance, Climb, Diplomacy, Escape Artist, Hide, Jump, Listen, Move Silently, Sense Motive, Spot, Tumble.
I could see exchanging Diplomacy for Intimidate perhaps. Your thoughts?
Planes of Law said:[Buseni] hide in the shadows and wait for some foolish berk to challenge their authority. Other buseni patrol the misty heights of Arcadia, their inner senses alert for any intrusion. Buseni always hover near the portal they’re assigned to guard, ready to question those who’d pass without permission.
If a person or persons unknown approach the gates to Buxenus, buseni step out of their hiding places at a time calculated to shock the travelers. Communicating nothing, their bodies suddenly form weaponlike protrusions – a warning display. If the approaching party chooses to attack at this little provocation, the buseni will do their best to kill them. The buseni reason that such lawless creatures would he better dead. If the group parleys with the buseni, however, they will respond, using telepathy to establish contact. Buseni will question the group about their reasons for wanting to reach the second layer of Arcadia. If the answers are satisfactory, the group must then remove all clothing, jewelry, and weapons so that the buseni can determine the alignments of the group’s members.
***
There’s no natural reason why buseni look the way they do, or why they are at all. It’s thought that they were spawned by the plane itself. Others think that buseni were created by some power eager to show that – despite a fearsome appearance – a creature need not necessarily be evil.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.