What does "saseru" mean in this sentence?
This is the sentence I'm talking about:
全身を武器に変化させる「変身能力」を持つ伝説の殺し屋。

I translated it as:
"Change the whole body into a weapon that holds a secret ability...(don't know here) legendary hitman/killer."

My friend translated it as:
"The hitman/killer who has the ability to change her body into a weapon."

My friend's answer makes more sense although from what I know '~saseru' means to 'let/make someone do something'.

P.S; There isn't much context to it, all I know is there is a girl involved... hence the 'her'

Thank you!

Question

What does "saseru" mean in this sentence?
This is the sentence I'm talking about:
全身を武器に変化させる「変身能力」を持つ伝説の殺し屋。

I translated it as:
"Change the whole body into a weapon that holds a secret ability...(don't know here) legendary hitman/killer."

My friend translated it as:
"The hitman/killer who has the ability to change her body into a weapon."

My friend's answer makes more sense although from what I know '~saseru' means to 'let/make someone do something'.

P.S; There isn't much context to it, all I know is there is a girl involved... hence the 'her'

Thank you!

Answer

「変身能力」 means "Transformation ability/Power to transform". 
"Saseru" when attached to a verb means "to let/allow one VERB".

Answer

Although you have already resolved the question, let me just add one thing. 
  
 '全身を武器に変化させる' is explaining 「変身能力」, and '「変身能力」を持つ ' is explaining 伝説の殺し屋. 
You probably mistook what explains what; that's why your translation make much sense.

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