So here goes Charon's second boon for sentinels:
On a Pale Horse (Su): You gain the ability to invoke the power of the Pale Horse. As a swift action, you cause a flickering image of the Pale Horse to appear in a 10-foot square adjacent to you. It then moves up to 120 feet as you mentally direct its movement (doing so is a move action that requires concentration). The image of the Pale Horse ignores difficult terrain and can walk over water or other surfaces that would normally not bear a creature’s weight, but it cannot fly. Any creature whose space the image of the Pale Horse passes through must succeed at a Fortitude save or take a –6 penalty to Strength, Dexterity, and Constitution, as if it had suddenly aged to become venerable. Creatures (like dragons) that normally gain benefits from aging do not gain any benefits, and they instead take the ability score penalties described above. These penalties do not stack with themselves or with existing penalties from aging. A creature can resist the effects of the Pale Horse with a successful Fortitude save (DC = 10 + half your Hit Dice + your Charisma modifier). A creature can be affected by the Pale Horse only once per round, regardless of how many times the Pale Horse moves through its space. This is an aging curse effect.
My question is: as there is no mentioning of a limited use per day, is it an at will power? If so, is it not OP (it comes at 13th or 16th level, levels which admittedly I am not familiar with)? Yes, it is limited by concentration, but you could simply "turn it on" whenever you like by spending a swift action.