Radiance FAQ: Baron Nashor Without a Pit, Mordekaiser and Atakhan, Riven's Timing

Riot has published the Radiance Rules FAQ and Clarifications on the official Riftbound site: 14 official answers on card interactions from the new set. Where the FAQ and the Core Rules differ, the FAQ wins until the next Core Rules update. There are no errata and no ban list changes.

The rulings that change how you play

Baron Nashor can be played without a Baron Pit for nine Energy and three Power with Hunters' Circle, because its cost reduction applies before his entry replacement effect.
Mordekaiser, Iron Revenant cannot banish the unit you kill to pay for Atakhan: Energy and Power are paid before that cost, so the unit is not dead yet. With the alternate cost of Meticulous Restorationist, instead, Mordekaiser can banish a unit or gear from your trash.
Riven, Broken Blade pays the buff cost of her exhaust trigger when the ability resolves. If it triggers together with Stellacorn Herder, Riven's trigger goes on the chain first.
Neeko, Blending In, played from your trash by an opponent through Kharox, does not give them the Legends you chose at deckbuilding.
Gravity Field: if you decline its cost, the Standard Move is undone and cancelled. With Charm you pay only if your opponent controls the Gravity Field at that battlefield.

Also in the FAQ

Mobilize still draws a card with one rune left in your Rune Deck, the play permission of Runeblade Anvil lasts until the Ending Phase of your next turn begins, and Possession works on Ekko, Revolutionary, who stays under your control when he returns.