I can add a bit to this one, but the lore here's not as detailed as the history behind the BotRK and armguard.
this cloak bears the crest of Shurima, and also looks a lot like the capes of Azir's soldiers. While this may be a stretch the cape is most certainly a Shuriman creation.
the newest item added to League, this thing spawns little voidspawn. The name Zz'rot is potentially the name of the Voidborn species these guys belong to or the name of a larger Voidborn who has history with the portal. I lean more towards the former here.

while the origins of many of these items aren't clear, they're all true ice weapons, meaning they had connections to the Frozen Watchers, the Iceborn and Freljord's long forgotten history. In particular
was wielded by an individual known as the Frost Queen, and we only know of two women who have ever held the title of queen in Freljord: Ashe, who is currently the Queen but vying for control of the land, and Avarosa, Ashe's ancestor, who united the Iceborn. Considering the situation it stands to reason the blade belonged to Avarosa a long ass time ago.
while its icon offers little
makes many references to it being battle-scarred and made for war, indicating that the Armour has seen its place in hundreds of battles. Some REAL old lore, I'm talking BETA lore stated that a troll warlord once wore the Armour and it kept him safe in many battles. Whether this is true is up for debate, I mean it's BETA lore, but hey, I say it's a cool idea to discuss!
another Freljord item exclusive to the Howling Abyss, this is the horn you see on the blue side shopkeeper. That's right, Greyor's horn is purchasable. It was made to be sounded by him should the Watchers ever crawl out of the Abyss.
Items are one of these things I'd love to see explored in lore, which they seem to be doing a bit of here and there, but lore about them is sparse and mostly comes from champion purchase quotes and the icons themselves. I'd love some item flavor text at some point, that's for sure!