Rhaast Theory and Darkin Concept
I only recently read the 
story so bare with me if this is late.
RHAAST THEORY
As we all speculate, Rhaast has significant potential to have been the Master of Vladimir during the Darkin reign. A Darkin that was cruel and disregarding of even his own kin, that was supposedly slain by Vladimir and hence gave Vlad his immortality. However, in Vlad's story we learn of connections to a 'scythe'.
Based on the actions of Vladimir's Master, teaching a human blood magic just to spite his own kin, I think it's also reasonable that he would have done other things to spite them. For instance learning how to trap his own brethren to wield as a weapon and steal their Sunborn powers from them. Unlike Ta'anari and Myisha's plan in Twilight of the gods, trapping the Darkin in weapons does not require all the Darkin to be in one place, nor was it possible to simultaneously trap them all across the world. Therefore, word would have spread of this technique of sealing the Darkin. Perhaps some tried and failed in an attempt on Vlad's Master and he took the spell to use for his own benefit.
However, things didn't go as planned and before he could use it on another Darkin, his warhost and Vlad turned on him. In the process of having his life essence drained Rhaast used the spell on himself escaping into his own weapon, unbeknownst to Vladimir or anyone else. Hence why Vladimir is only 'partially' transformed, hence the scythe banner and hence why Kayn is Rhaast's first real host, he's been in a weakened state until recent, due to having been drained.
DARKIN CONCEPT
A lot of people have speculated/ are hoping that the next Darkin will be mutualistic in nature and not parasitic like the other Darkin. Seeking redemption and acknowledging the cruelties they committed. To that I say "Nah... what if they were 10 times crueler than the ones we already know of :D".
A Darkin so cruel they preyed on other Darkin, going around sealing their own kind, not to put an end to the war, nor to aid the people of Runeterra, but for power. He sealed them and absorbed their power, similar to what Shadow Kayn does to Rhaast, a Darkin that wielded numerous 'Darkin weapons' an Arsenal... Upon being sealed himself he realized that because his soul was tethered to so many weapons, from absorbing different Darkin, he could move freely between the weapons. Now he is back on Runeterra and in search of his 'Darkin Arsenal' to wreak havoc once more.
TLDR: Rhaast sealed himself within his own scythe and A Darkin with multiple weapons.
Thanks for reading, let me know what you guys think.
