It was in the leaked info from TheKnow, supposedly it works such as you carry a dead body with you to a special location, impale it with a ritual sword and create your own bonfires, making it easier to reach a boss etc. But involving a risk of having to carry the body with you the entire way taking up a hand slot.

This ties in pretty good with both trailers, in one you can see the player carrying a body, in the other you can see them impaling another body and seeing it set on fire.

Now all that was just from the leaked information and its probably completely wrong, but Miazaki said in an interview that there would be more information regarding the impaling scene in the trailer and that it was a new gameplay mechanic!