Is that her heart?

nope after his lose in LotR sauron was sent too hell and got stuck "there"

wizzard87 said:
nope after his lose in LotR sauron was sent too hell and got stuck "there"

Seems more like a reward than punishment.

No, its...uhh...well, its hard to explain, but she has that thing lodged in her chest, for various reasons.

Okuu is a former Hell Raven who was upgraded to a Yatagarasu by divine power.

Yatagarasu have three eyes and three legs. That thing in her chest is an actual eye.
(And yes, it does mean she has her shirt half-open in all official artwork.)

The "third leg" is her arm cannon.

Which, incidentally, is not actually pictured here.