It's a little action-packed, but is definitely the best example of PKB and Heart's love. Near the end, Heart strangles one of The Three Dark Ones, but she ends up drowning due to her actions. PKB gets so angry he goes insane and starts beating up Dark Question, the last surviving Dark One in a way nobody else has before or after in the series.
And apparently killed him, as he shows up in The Three Light Souls in the afterlife. Either that or he drowned in the ensuing flood.