One of the mechanics that I've used for a few of my games is creating bound entities that the players have control of/access to that they can draw power from but must, first, perform some act of obeisance to or act antagonistic to the very nature of the bound entity in the first place to gain the resources to use/access said nifty abilities. For example, one of the players has a Balor bound to a dragonshard embedded in their chest. In order to gain control over the bound Balor, the player must take actions that are antagonistic towards a creature of chaos and destruction, specifically, when rolling initiative, losing the ability to score critical hits until the end of the encounter and/or gaining the pacifist attribute (if you hit and deal damage to a bloodied target, you are stunned until EoNT) until the end of the encounter. Said acts are antagonistic to the bound creature and provide a resource in the form of "control", which the player can then expend to gain various effects, like gaining the Balor's fiery aura for a turn, gaining access to magical empowerment of the Balor's Vorpal Sword (i.e. massively expanded crit range) for a short while, or being able to create a magical whip of fire to pull an enemy (or ally) close. The way I designed it, the antagonistic acts or acts of obeisance are *very* debilitating for that encounter (the character with the Balor is a dwarven Slayer Fighter who loves getting crits and wrecks enemy's faces) and the abilities themselves are *strong* but weak compared to the cost of what you had to give up to gain said control (just consider the whole "expanded crit range for one turn", which I have cost 3 control, to the "you cannot score crits" which provides only 1). On top of that, if the character ever dies, the bound entity is released and decides to get revenge upon the party right then and there, which can *really* suck for the survivors (which has happened; in a fight that was supposed to be reasonably easy, the players chose to let lose with a large number of said acts of obeisance and, combined with some bad rolls, ended up having the dwarf die and turning a +1 fight into a +4 fight thanks to the addition of the Balor; the party survived and managed to resurrect the dwarf and rebind the Balor, but it took all of their dailies and encounters, along with one player completely bottoming out his substantial pool of control, to survive it). It creates an interesting resource to play around with and definitely has aspects of "awesome but impractical", though I definitely tried to make it relatively practical. You could play up the impracticality of it by making the obeisance/antagonistic acts more severe. Of course, the *simple* way to do awesome but impractical is to provide an *awesome* weapon (like, say, a Vorpal Sword) that also happens to be cursed (with the curse of, say, Backbiting): the sword will wreck face but do you *really* want to risk having the sword decapitate you while you're hewing through hordes of orcs?
