A very cool moment as Ragnar, still very much a sickly man (that part wasn't a trick), pumped up his intensity for one final act. And that shot of Bjorn throwing up his arms, sending the vikings in to plunder the streets, was awesome.
Answer Wiki. Incubus is a mythical creature, a demon in male form who preys upon sleeping women in order to father children. According to Malleus Malleficarum, demons are infertile and unable to procreate (due to the fact of them being dead) so they came up with a proxy way of making people.
However, they also exist in the physical world. They gather energy via sexual acts to channel back to the succubian horde. The succubian horde is the grand collective of demons, both incubi (male versions of succubi) and succubi (plural of succubus) take part in this demon circle.