Anu was one of the very first beings that "was." He's stasis and his "brother" is Padhome, who is change. Their interplay essentially created the Aedra and Daedra.
Arden-Sul, from what I gathered, is basically a "head honcho" of the Shivering Isles who basically poisoned some liquid and gave it to partygoers at a hedonistic gathering, effectively exploding their hearts. Somehow Arden-Sul's heart was removed as well; I'm sorry but I don't really remember all of the details at thsi point in time.
I haven't heard of Mystara yet; I shall find out.
Arden-Sul, from what I gathered, is basically a "head honcho" of the Shivering Isles who basically poisoned some liquid and gave it to partygoers at a hedonistic gathering, effectively exploding their hearts. Somehow Arden-Sul's heart was removed as well; I'm sorry but I don't really remember all of the details at thsi point in time.
I haven't heard of Mystara yet; I shall find out.
No, Anu was the very first "being"...
He then created, if not allowed, Padomay/Sithis. Who, in theory was the exact opposite of Anu. Anu being nothing really at all, so good in a sense. Padomay being the breath of life(?), bad in a sense. The Dark Brotherhood see him/her as a symbol of all things, such as the act of killing. Whereas Anu gave life to Padomay, Padomay/Sithis takes them. Killing...
Edit:
Of course, this isn't to say Anu is good and Padomay is bad, just that, where you find one thing, you will find the exact opposite.