Mystical Source

Mystical Source was a legendary crafting material used to create pieces of the Sage's Journey set. It is account bound, has item level of 1, and can stack up to 1000. As of patch 2.0.6., it no longer drops, and existing Sources can be sold for 50,000 gold.

It could be dropped by unique Unicorns (Creampuff, Team Unicorn, Nightmarity, Super Awesome Sparkle Cake and Midnight Sparkle) in Whimsyshire.

Awfully whimsical.