In Fortnite’s Fortnitemares, you can only respawn as a Shadow after getting eliminated. There is no unique trick required to complete this task. Whenever someone eliminates you, you will respawn on the Battle Bus and drop into the match as a Shadow.
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Once you respawn as a Shadow, you can play as a Shadow untill either match ends on your killed by another enemy.
If you are finding it hard to know how to get killed and complete Fortnitemares challenge quickly, building a few ramps or jumping off a skyrocketed building or high mountain is highly recommended.