I don't remember off hand if there are any Demons listed in the DMG's Leadership special Cohort related tables / texts.
If any demon has an LA you can use it.
If not, you'll have to convert a demon into something like what the dmg lists.
You get the demon at level 6 or 9 or whatever.
It is the "level" it is, whether legit or houseruled.
Then it gets to gain levels (a little slower than you).
That's when it takes Fiend of Possession class levels.
'nother houserule possibility ...
count a Tiefling as a Demon.
Further afield, just charm/dominate/thrall the demon, PAO it into the sword, might wanna hit it with a Feeblemind,
just so the Ego score is a bit lower.