Hmm... Short of adding stuff how Seto rode a fearsome beast to battle to Seto's deity write-up, I don't think there's really a way to make it fit. So how about this alternative ability in its stead:
Skill of Fear(Ex): A sixth-level Grim Reaper learns new ways of using fear to his advantage. He can use the Intimidate skill with the following three options in addition to those normally available:
Attention of Fear: With a succesfull Intimidate check as a full-round action that doesn't draw an Attack of Opportunity, a Grim Reaper can hold a creature's attention. The target creature suffers a -5 circumstance penalty to Listen, Search and Spot check as their attention is inadvertantly drawn towards the Grim Reaper. This penalty doesn't apply agains the Grim Reaper himself. This effect lasts for a number of rounds equal to one-half the Grim Reaper's Charisma score, rounded up, or as long as the Grim Reaper remains visible to the target, whichever is less.
Faith of Fear: If the Grim Reaper acceps a -10 penalty to his Intimidate check while influencing an NPC, the NPC can be treated as Helpful instead of Friendly on a succesfull check. The NPC remains Helpful for 1d6 plus the Grim Reaper's Charisma bonus minutes, after which the NPC remains friendly untill 1d6 x 10 minutes after leaving the Grim Reaper's presence.
Lies of Fear: The Grim Reaper can choose to make an Intimidate check at a -10 penalty instead whenever he would be required to make a Bluff check, including feinting in combat. All normal modifiers for the believability of the bluff applies to the Intimidate check as do the normal modifiers for an Intimidate check, including the target gaining any save modifiers against Fear effects on its Sense Motive check.