Unlike some uniques tied directly to boss drops, Dragonsong is a random world drop, which means it can drop from any loot source within the game, though some methods give you better odds than others.

Rotting Aspect can only be used by the Necromancer class. In Diablo 4, the strength of affixes are randomly generated within a range of values. This is shown in [parenthesis] in tooltip above.

He looks back fondly on the times he visited cyber cafes, watching the older boys playing the game and shouting, "Master Chief!" If given a chance to inhabit a game world, he would eagerly jump ...

Aspect Of the Unwavering can only be used by the Sorcerer class. In Diablo 4, the strength of affixes are randomly generated within a range of values. This is shown in [parenthesis] in tooltip above.