Before we get too excited on Angelblade and Demonblade
They are really related to the new Missions coming soon in league. If you don't know what Missions are here is a link to what it is. TLDR; It's like the quests you do in Heroes of the Storm, where if you do a task like play Draven 10 times, you get a small reward, more information in the link provided.
In terms of Angelblade and Demonblade, their are a few assets other than the art and the icons that relate to both missions and the skins.
No one is talking about these assets right now. For me at least, they look like collectibles, and they're in the same aesthetic of Angelblade and Demonblade.
My guess, which I maybe completely wrong, that to get the skins for free, you need to collect a certain amount of these collectibles and craft it? Like Hextech Annie and Dreadnova Darius That's what I'm seeing right now. Of course it's just a guess and they may release it with RP. Whatever they are, I'm just excited for the new missions!