Simplify the following text for a Grade 9 native English-speaking person.
Original: Rumors of a posthumous Nirvana box set , or anthology , first surfaced in the mid-1990s , not long after the death of the band 's singer and guitarist Kurt Cobain in April 1994 .
Simplified: It was the only box set by Nirvana , and it one of several albums by Nirvana after the band broke up because the lead singer and guitarist Kurt Cobain died in 1994 .