Loot and attunement
Classic: Attunement to the Core (BRD quest) grants teleport via Lothos Riftwaker; walking in through BRD needs no attunement. Aqual Quintessence needed to douse runes for Majordomo. Forever: no attunement info; not in BlizzCon Legacy Challenge raid list (only Onyxia, Hyjal Summit, Barrow Deeps shown).
Description
Ragnaros's elemental lair beneath Blackrock Mountain.
Forever evidence
Clay Stone (What's Next panel): 'The original iconic raids that you know and love have not opened yet, and we're excited for you to rediscover them in the future.' Not named individually; could be the Summer 2027 revamp candidate (speculation only - forum users favour Molten Core).