It's already been said enough times, OOC reasoning and such, so I'll just skip ahead.
Summer's all about growth, right? Well then, their Slate-style torture could be just as slow and just as painful.
Just to start it off, you've got options, loads and loads of 'em. Top of my list both involve events hitting the fan just after the portal closes. Drop them in forest, fairly nice and slightly warm, like a balmy summer with a light breeze blowing and everything. A short ways off they can hear the baying of hounds and the trample of hooves at gallop several Fae cavalry charge them with nets and lassos accompanied by Fae hounds, intent on capturing the trespassing Oathbreaker and bringing him to Summer's stronghold.
Other opening option I'm fond of has the offending PC ensnared/cocooned by a sudden upsurge of roots, leaves and soil, pulled into the ground and transport to said Stronghold.
From there, the torture could proceed along two paths. One plant-based where the victim is pinned in place by vines as strong as iron and a multitude of vine-like and other plants burrow into him. And one biomancy-based where he is slowly tortured by some terrible applications of Seelie-powered biomancy to induce cancer and sickness (promoting that kind of growth inside him) while likewise bound in place. Both follow a simple rinse and repeat cycle until the other PCs affect a rescue attempt, or the powers of Summer choose to kill him.