Rules FAQ
If Second Sons is put into play by Flea Bottom, at the end of the phase the Forced Interrupt on Second Sons will resolve before the delayed effect of Flea Bottom, so Seconds Son will move to the discard pile, not the bottom of the deck.
Mercenary.
Bestow (3).
Forced Interrupt: When the challenges phase ends, sacrifice Second Sons unless you discard 1 gold from it.
Link: Decklists
Rules FAQ
If Second Sons is put into play by Flea Bottom, at the end of the phase the Forced Interrupt on Second Sons will resolve before the delayed effect of Flea Bottom, so Seconds Son will move to the discard pile, not the bottom of the deck.