Witcher 3 import save game
In Wild Hunt you will have an interview with Nilfgaardian general Moorvran Voorhis during the early story mission Imperial Audience. during that conversation you have to answer few questions about events from "Witcher 2" that can affect the "Wild Hunt" storyline - one of those questions is the fate of Leto.
If you'll say that you've spared him, you will be able to meet him and after doing short quest, ask for his help.
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