The Yggdrasil flower is a key item unique to Dragon Quest IV for the purpose of reviving Rose. The flower blooms after the party defeats Foo Yung and Chow Mein in the bonus dungeon below Azimuth. However, the party must still travel to the top of the Yggdrasil tree to collect it. The item may only be used on top of Rose's grave in Rosehill.
The description reads: "The rarest of flowers that blooms but once every millennium."