The Clash of Titans

The Infernal Six against the Royal Flush: the parties of Curse of Strahd and Cards From Beneath, set against each other.

All six duels at once on a giant card table, each pair on its own playing card, Strahd and Valis looming over it as the two dealers

The full clash on a moonlit shore below a castle and a lighthouse, both parties charging with magic of every colour colliding

The sides

The Infernal Six

Myra, with her spellbook and divination, and the wolf beneath.
Taara, with her bow and her swarm of butterflies.
Khione, blind, with her glaive and her glory.
Mark, with the Cleaver of the Last Cut and the severed hand.
Logi, with wildfire, the Bloodspear and Cinder.
Yager, with the green flame sword.

The Royal Flush

Percival Vale, with eldritch blasts and his Form of Dread, and Valis behind him.
Gia, with Fury and Grief and her Vayne revolver.
Onyx, Liora and Nessa.

The fight

A duel of magic in a ruined chapel: Myra's purple sigils and Taara's butterflies against Percival's eldritch blasts and Valis's yellow claws

Steel on a harbour street: Mark's cleaver against Gia's twin daggers and revolver, Khione's glaive and Yager's green flame sword in the fray

A burning forest: Logi's wildfire and a blazing Cinder against Percival in his Form of Dread

The standoff before it starts: both parties across a gambling table, weapons half drawn, Valis's eyes in the dark above

Duels

Strahd against Valis above the castle spires under a blood moon, bats and blood magic against a faceless giant with a broken halo

Percival firing eldritch blasts from a rain-slick rooftop as Taara flies above with her bow and a swarm of blue butterflies

Gia with Fury, Grief and her revolver, crumbling into sand, against Myra's spellbook and purple sigils, her wolf shadow behind her

Liora rising from the surf with a whale's shadow in the wave, against Mark swinging the Cleaver of the Last Cut Onyx glowing with spreading blue light against Logi thrusting the Bloodspear through a wall of wildfire, Cinder leaping between them

Nessa striking from the dark with the ghosts of those she killed behind her, against Khione's radiant golden glaive in a lighthouse

All duels at once

All six duels across a moonlit shore between Ravenloft and a lighthouse, Strahd and Valis in the storm above

All six duels on the levels of a ruined cathedral under a blood-red sky, Strahd and Valis above the rose window

All six duels across the rooftops, bridge and pier of a foggy harbour town, Strahd and Valis over the roofs

All six duels in a ring across a stormy moor under a sky split between blood moon and golden void

Still open

This has not happened at either table. Where and why the two parties meet, and who wins, are not written.