This card has been discussed in a few places, but those were mostly thread-jacks, so I thought it deserved it's own thread with compiled discussion.
Obviously you can cast this on your own Warlock and try to ensure a friendly demon creature attacks an enemy creature each round. And this can be combined with Demonhide Mask to make your Warlock's own attacks (including attack spells) count to activate Eye of Bael.
But an interesting thing about this spell is it isn't Warlock (or Dark Mage) Only.
... I would probably also cast Eye of Baal on my opponent to try and speed up the damage rate.
Now, before you get too excited, like I did, there are a couple of caveats when it comes to casting equipment on an enemy creature. Here's what the Rules Supplement has to say about it:
Equipment
Equipment Control
The Mage to whom the equipment is attached controls that equipment. For example, if you cast Sectarus on a friendly Mage, he will be able to attack with Sectarus, and plan curse spells for it. This is different from other spells, such as enchantments and conjurations, in which case control is retained by the caster of the spell.
Note that you cannot cast, control, or use equipment which you are restricted from. For example, A Priestess cannot cast, control, or use a Staff of Beasts which is Beastmaster Only. See “Spell Restrictions”.
Replacing Equipment
You cannot have 2 pieces of equipment attached which have the same name or the same location. For example, you cannot have 2 Leather Gloves attached (same name). You cannot have both or a Bearskin and a Dragonscale Hauberk attached (same location).
If an equipment object of the same name or location is cast upon a Mage, if the spell is friendly controlled, the new equipment object is placed on the Mage, and the duplicate item (the original one which has the same name or location) is removed and placed in its owner’s spellbook. In this manner, friendly Mages can replace equipment on each other with newer or better choices. This might be useful for removing equipment armor which is no longer useful because it is corroded (see “Corrode”).
Note that if an enemy-controlled equipment object of the same name or location is cast on a friendly Mage, the new equipment object is destroyed and placed in its owner’s discard pile. Enemies cannot replace equipment items on friendly Mages.
At first I interpreted this to mean that, if your opponent puts an Eye of Bael on you, you can't simply replace it. But on re-reading, it would seem that isn't the case. If an enemy tries to replace your equipment (e.g. replace your Moonglow Amulet with Eye of Bael), the new equipment (EoB) gets destroyed and goes to its owner's discard pile. However, if your opponent put EoB on you first, you "control" it, so you can replace it with your own Moonglow Amulet, causing it to go to its owner's discard pile.
At least, I think that's how it works.
That said, if your opponent doesn't have an amulet to replace it with, their own option is Dissolve/Crumble (or Explode or Corrosive Orchid), or be stuck with it for the rest of the game, making it a cheap curse to play.
Too bad it doesn't have the Curse subtype.
Have you had success with Eye of Bael?