The first thing to get straight
Search for "Forsaken Kingdom" and Forever together and you will find people describing a Forever campaign. It is not one.
Forsaken Kingdom is a campaign in Warcraft III Reforged. It follows the newly freed Forsaken as they fight to establish themselves beneath shattered Lordaeron.
Its relevance to Forever is a matter of timing, and the timing is precise.
Where Forever sits on the clock
The wiki puts it in one sentence: Forever "is indefinitely set in the first year of World of Warcraft (25 ADP), after the events of the Forsaken Kingdom campaign in Warcraft III Reforged: Forsaken Kingdom but before the events of the Molten Core."
That is a narrower window than "Classic, but more." It has a floor and a ceiling:
- The floor is the Forsaken Kingdom campaign. The Forsaken already exist as a power. Undercity is already dug.
- The ceiling is Molten Core. Ragnaros is still down there, unbothered.
Everything Forever adds has to fit between those two points. That is not marketing language - it is a constraint on what can appear.
What the campaign covers
The short version of the underlying story: Arthas Menethil destroyed Lordaeron and raised its people into the Scourge. Sylvanas Windrunner, freed from the Lich King's control, gathered the undead who had also broken free, named them the Forsaken, consolidated a hold over the ruins of the capital, and built the Undercity beneath it.
Forsaken Kingdom is that consolidation played out - the fight to turn a population of escaped corpses into a state with a capital.
Why Forever cares
Because Forever puts a dungeon there.
Ruins of Lordaeron is one of the nine new dungeons, at level 15 to 20, and it uses the actual ruins rather than a new location that resembles them. For Horde players it arrives extremely early - a group can come out of the racial starting zones and into a new instance before level 20.
A dungeon set in the capital the Forsaken just took, in a game that begins right after they took it, is the clearest signal Blizzard has given about what "new storylines in the old world" is supposed to mean. Not a new continent. The same map, a few years on, with the consequences still wet.
One naming trap
Players who came in later know Ruins of Lordaeron as a PvP arena above the Undercity, introduced in The Burning Crusade. Forever's Ruins of Lordaeron is a levelling dungeon and has nothing to do with it. Arena guides under that name will not help you.
What is not known
Whether Forever picks the story up or just borrows the address. Blizzard has not said whether Sylvanas appears, whether the campaign's characters carry over, or whether the dungeon's story continues anywhere.
Nor has anything been said about how much of the campaign a player is expected to know. Forever is being sold as a place to start over; a starting point that depends on a Warcraft III campaign is a strange thing to leave unexplained, and so far it has been left unexplained.
What the dungeon itself contains - encounters, quests, loot and how firm any of it is - is on the Ruins of Lordaeron page.