Sadde had thought they'd felt the worst they could possibly feel after they killed their mother. In hindsight, they obviously should have recalculated after the utter undiluted indescribable joy meeting the triplets, Katur, and Kaede was.
They knew something was wrong long before they reached the portal. They could see, in the clear desert air, the accelerated heartbeats, the panicked looks on their mates' faces. Sadde heard their words, telling them to fly faster, harder.
They had been almost, almost there when the portal closed.
Their brain refused to process what had happened, at first. A lot of space for thought also means a lot of space for denial. They relived those few seconds several times, but the playback always stopped before the portal closed. They couldn't, wouldn't accept it. Maybe if they relived it enough times, fantasised about enough different possible outcomes, one of them would come true. Other universes existed, right? Who's to say they couldn't make themself be in another universe, one where the portal had never closed, one where this hadn't happened. Anywhere but there. Any reality but theirs.
They didn't notice the invisibility spell run out.
Eventually they gave up. There was no point denying the pain truth of their senses: the portal had closed.
They never went through anger. Such a useless emotion. Instead, they skipped straight to bargaining. They weren't completely powerless; they were a sorcerer, too, one with an eidetic memory, and they'd seen the portal ritual. They could do it, too. They had more than enough lifeforce.
They set to it. They found an appropriate place, they found the necessary resources, they performed the ritual. The ritual needed a focus. They used themself. It didn't work; they weren't from Elsewhere. So they tried modifying it, figuring out a way to do it based on their memories, on their brief presence there, on their connection to the triplets and to Katur (and to Kaede, even Galatea with just one of their mates would be better than being alone).
(Their mates were separated, too, oh god, Sadde couldn't even take solace in the fact that they all had each other.)
None of it worked. None of it worked no matter what they did. They spent two weeks trying, and none of it worked.
They contacted the Golden Coven. The Emperor could read their mind, the Princess could verify the truth, Marcus could see their mate bonds. The Coven was of course fascinated by the fact that it was apparently possible to mate on multiple people under sufficiently contrived situations, and updated their pamphlets accordingly, but this didn't actually help Sadde in any way. Razi (the Imperial Teleporter) couldn't teleport there, Alice (Imperial Seer) couldn't see other worlds, Nathan (Minister of Temporal Affair)'s best time to try various things didn't actually yield a good time, Siobhan couldn't plan about it.
Multiversal resources were too good to be wasted, though, so even if not for purely altruistic purposes (or not purposes purely related to Sadde and their mates, they supposed their reasons were in fact altruistic) they helped. Addy (who had become Imperial Factotum) was particularly interested in sorcery, but it proved hard and slow to research, especially with a less than enthusiastic Sadde being its sole holder.
Sadde at some point offered up the fact that they'd tasted and absorbed some delicious synthetic blood and could reproduce it for the Coven to try to reverse engineer it, and to offer up at capitals. They started travelling with the moving capitals for this, a permanent fixture - they were even offered an Imperial title, but had no interest in it. They had no interest in anything. They produced blood. Eventually they started wrangling newborns, too, although not with Jasper (Imperial Newborn Wrangler), who found being around them way too depressing. They were pretty sure they'd never move on to acceptance.
They asked the Emperor how he'd dealt with thinking the Empress was dead. His answer was useless. They asked the Empress; she kept thinking that the Emperor would have wanted her to live, and then had the Princess to raise. They asked Jasper; he'd gone crazy. They asked Marcus; he looked at them for a few seconds and eventually said "Chelsea." They stopped asking after that.
They travelled. They opened doors, everywhere, obsessively. Whenever they had free time, whenever they could. Milliways had become their only hope. They were immortal, they could do this forever. They knew their mates would be immortal, too, so they'd eventually find each other. That kept them going.
But "going" was the best that could be said about them.