Beings of Tash Kalar
The pattern resembles the priestess figure in her robe. Unfortunately, on the picture, the robe is covered by the textbox.
The priestess slowly approaches a common piece and blesses it. "Rise from your knees, commoner. From now on, you are known as a hero."
As you would guess, the High Priestess is great for preparing patterns for Legendary beings. And useful for tasks that require upgraded pieces. However, she is not very good at disturbing enemy plans, so if the game gets too competitive, you should summon her as soon as possible to get a strong offensive card into your hand.
Note: In melee, even the High Priestess can do some damage. If you use an enemy token as the top of the pattern (through improvised summoning), you can destroy even a heroic piece with the High Priestess.
After the decks were split, the High Priestess was put into the Imperial deck along with a card called "War Priest". Both were rather complicated cards (the High Priestess originally both converted enemy pieces and upgraded your own under certain conditions, and the War Priest was both destroying and converting enemy pieces under certain circumstances). However, the effects were hard to imagine and they slowed down the first games, so we decided they were not suitable for Imperial decks. Also, we wanted converting to be a Sylvan trademark ability.
In the end, the War Priest was dropped and the High Priestess got an effect that is easier to imagine. The ideas are not forgotten, though, and maybe sometime in future... :)