Simplify the following text for a Grade 4 native English-speaking person.
Original: BrÃ 1\/4 nnhilde , seeking revenge for Siegfried 's manifest treachery , joins the plot and tells Hagen that Siegfried would be vulnerable to a stab in the back .
Simplified: Hagen tells him about BrÃ 1\/4 nnhilde on the rock . Siegfried has forgotten about her .