I would give them the ability to invoke a specific patron (probably making a designer table) but not grant patron bond in this case. That way, they would not gain the ability to bond others to that patron, or to bond to other patrons. I would also rule that access to the spell is not the same as learning the spell, so it would not use up a spell slot, and would require the caster to keep the horn on his person.
That's probably a better way to do it. Maybe it's a very limited form of invocation, perhaps only really useful for summoning similar demons (which, if the user isn't prepared, could end up being highly entertaining...).
Hamun Ry (Wiz 4)
Str 10 Agi 15 (+1) Sta 11 Per 11 Int 17 (+2) Luc 10 (Unholy House). Align: C. AC: 14. HP: 13. Melee +1, Ranged +2. Crit: d8, I. Save: Ref +5, Fort +3, Will +4.
Spells: 1: Choking Cloud, Comprehend Languages, Detect Magic (odd crystal growths), Magic Missile (mirror images), Runic Alphabet (Mortal) (ravenously hungry), Ventriloquism (rain of frogs)
2: Detect Invisible, Levitate (extremely difficult, d14), Mirror Image (20% chance to raise/lower luck by 1d3 points).
Equip: Ring of the Sand Djinn: +2 AC/saves, Invisibility for 1min/spellburn point, or unmake for great, unknown effect. Padded Armor, Longsword, Longbow, quiver w/20 steel-tipped arrows, 10 silver-tipped arrows, backpack, spellbook, quill and ink, sturdy parchment (10 sheets), 5 days rations, high leather boots, belt w/ belt pouch, gray robe, dark gray hooded cloak. Also carries 3 small mechanical toys: wind-up mouse, wooden puzzle cube, small jewelry box that plays a little tune.