Lyric discussion by binni 

Cover art for The Swan Song lyrics by Within Temptation

It's called "The Swan Song" because a person's final act or effort is often referred to in that manner. (Taken from the old belief of the Mute swan singing right before it dies).

Evanescence's "Lithium" has nothing to do with Nirvana's. Amy is using lithium as a metaphore for numbing or repressing one's feelings (good and bad).