Obtain From:
Item: [Ancient Ruins Orgg Reward Bundle]
Item: [Ancient Ruins Saunil Siege Captain Reward Bundle]
And drop at end of Quest: [Co-op] Ancient Relic Crystal

How to get:
Get 5x [Ancient Relic Crystal Shard] (by fishing or rare drop at high mobs (Treants, Shadow Knights etc.)
Arrange them as a + in your inventory,
and you will get [Ancient Relic Crystal Summon Scroll] (beware, will disappear after 7 days, lvl 50 required)
Summon the Relict and you have to fight against 3 Bosses with adds (better to do it with friends)

Here is a short video how to farm them overcoming over contested spots
https://youtu.be/CTbn3YbbThE