Well I just recently caught up with all the storyline thus-far and wanted to jot down my thoughts about Icehearts ideals, and how she was able to evoke/channel Shiva by using herself as a vessel- and even surviving the ordeal after Shiva was defeated.

From what we know so far, Iceheart is a vandal and is very much against the current corrupt Ishgard rule- as it was shown in the trailer, the Archbishop is being practically puppeteered by the Ascians- the already divided nation becoming spread even thinner as the Iceheart heretics work on all ends as spies.

we can't be sure if they're loyal to any of the families yet. But she is clearly also blessed by Hydaelyn, aswell "hear, feel, think" and being capable of transporting herself to the light side of the rift, she may be a separate entity, similar to. but not quite like the Ascians.

I'd be delighted to hear more variation on theories- if there may infact be some more solid proof I've not yet dug up-