When Blizzard launched the Diablo 3 2.6.0 patch they also launched the new Challenge Rifts and this is how you can test the Necromancer for free.
Every so often, Blizzard will change the build that’s being showcased in the Challenge Rifts and the Necromancer is the class that’s currently being highlighted. By taking on the current Challenge Rift, players can play the Necromancer class for free. There’s no need to own the Necromancer Pack at all, the class is simply available to try. Blizzard points out that “it will be a while before we feature another Necro build” so take this opportunity while you can.
You can read our thoughts on the Necromancer Pack and then try it yourself right now, just make sure your Diablo 3 is updated to v2.6.0.