Not gonna pretend I've ever cared to understand sc properties extensively or anything, but why does Evis>Rudras>Rudras make darkness x2, but Rudras>Rudras makes darkness but then a 3rd Rudras doesn't make darkness again? Is the first one T3+T4 Darkness or T3+T3?
So whenever skillchain properties go onto a monster there's a limit to how many effects can be applied in a "set" of skillchains.
Iirc, the cap on it is 7.
Each tier of skillchain takes up an amount equal to their tier.
So Evisceration > Rudra's = Darkness. They make the Darkness with Gravitation and Distortion. These are the SC properties that are applied to the monster's condition. So it's received 4/7 of its skillchain set allotment.
You may then perform a second Rudra's, Mercy Stroke, or any other WS with a Darkness primary property, to cause a second Darkness skillchain to result, as the set caps out at 7/7.
Meanwhile Rudra's into Rudra's results in 6/7 singular Darkness, and can't be SC'd off of, to my knowledge, because there's no tier 1 skillchain that can form a skillchain off of Darkness.
There's a bit of a loophole too with multistepping.
For instance if you take the skillchain set of:
Full Break -> Upheaval = Fusion -> King's Justice = Light -> Ukko's Fury = Light It actually allows for an over capped situation.
You reach 6/7 on King Justice's Light, and the game allows the creation of a skillchain from Ukko's because it's nice to us, so you overcap at 9/7 and the set is complete.