Rifts
A rift is a tear in the world itself, not a door you walk to. Step through one and you get a descent nobody has run before: the floors, the monsters, and the thing waiting at the bottom are all built fresh for that rift alone, so the same rank never plays out the same way twice.
What a rift is
Dungeons are places. They sit where they have always sat, and you learn them until you know every corner. A delve is a short private descent you start from a board, cut for one or two. A rift is neither: it opens on its own, out in the world, with no warning, and everything inside it is generated the moment it does. Nobody has a route to hand you, because nobody has been down this one. It is instanced like the other two, so what you find inside belongs to you and your group alone, but it is the only instanced content in the game that comes looking for you rather than waiting to be found.
Where rifts open, and how often
Rifts tear open out in the wider zones of the realm rather than in the early valley, and the whole realm hears about it: a line in your chat names the rank and the zone the moment one appears. Each eligible zone comes up for a rift about once an hour, and a zone that already has one standing waits its turn rather than opening a second. A rift nobody closes collapses on its own after a couple of hours, and the realm hears that too. A zone whose rift was sealed stays quiet until its next turn comes around, so an announcement is worth walking toward while it is fresh.
The four ranks
Every rift is ranked C, B, A, or S, and the rank rides in the announcement, so you know what you are walking toward before you leave town. C is the gentlest, B and A climb from there, and S is the hardest thing a rift can be. Rank is the dial that decides how mean the floors are, and the harder ranks pay accordingly, so take the one your group can actually hold.
Who goes in
Bring a group. A rift does not soften because fewer of you walked through it: nothing inside counts how many are standing there, so the rank on the portal is the rank you fight, whether that is five of you or one. The doorway will let you in alone, and people do try it, but a rift is group content at every rank and it is honest about that from the first room. Your party gets its own copy of the rift, so no other group can spill into your run. If you fall, you can walk back in as a ghost and collect yourself once the fighting inside has stopped.
Rifts are endgame content. You have to be at the level cap, level 20, to step through one, at any rank.
Down through the floors
A rift runs a handful of floors, each one built fresh, and the way down does not open until the floor is finished with you: clear what is living on it, solve whatever it has locked across your path, and the descent tears open. The last floor ends on a boss. Every floor takes a character of its own, so a single run can carry you out of the frost and into the embers, and your chat names each floor as you arrive on it. The tracker on your screen is where you learn how many floors this one has. A few rifts open onto a hand-built set piece instead of a generated descent, which runs to its own fixed length.
Once you draw blood
The first kill your group takes inside a rift settles the run, and so does the first off-path cache you crack open. From that moment you are bound to that copy of it: step out for any reason and coming back puts you in the run you left, never in another group's and never in a fresh one, for as long as the entrance out in the world is still standing. Before that first kill, and before that first cache, nothing is settled, so a group that is still gathering can regroup and walk in together without stranding half-started copies behind them.
The race for the first clear
Every group in the realm can attack the same rift at once, each in its own copy, and only the first to bring down the thing at the bottom seals it. When a group wins, the realm hears their names and their time, and the way in closes behind them. Losing the race does not end your run: your copy stays open, the thing at the bottom still falls to you, and you still walk out under your own power. What it costs you is everything that clearing it would have paid. The boss leaves nothing behind for the group that came second, so what you carry home is what dropped off the mobs on the way down, and nothing more. The Book of Deeds still counts the clear, because you did put the thing down. It is the only race in the game you can lose without ever laying eyes on the people who beat you.
What you carry out
Sealing a rift, not merely surviving one, is what pays. Bring the rift down first and it pays like the instanced content its rank stands beside, so the harder ranks are worth the harder run. Sealing also puts a Riftbound band in the hands of everyone who was there, cut to your class's role and personal to you, and leaves Rift Essence in your bags besides, with rift gems on top of it at the harder ranks. Beside the way home, the thing at the bottom leaves a sealed cache your group can pick open for extra spoils, using the same Tumbler's Path lockpicking you know from delve chests, so a clean, patient job pays better than a rushed one. None of that reaches a group that came second: a lost race leaves you only what dropped off the mobs on the way down. The Book of Deeds is the exception, and it counts your clear either way, with a deed for closing your first rift and another for taking down an S-rank one.
The Rift Forge
The band a ranked first clear mints is not finished when you receive it. Riftwright Maelis, who keeps a forge in the Watch Meadow on the Farshore, up the shore from Gullhaven beside the Breach Scholar, will raise its item level one step at a time and set the coloured gems the rifts drop into its sockets, each colour one combat rating. A full band takes a new gem in place of its oldest, so you can retune it later. All of that is paid in Rift Essence and Rift gems, the forge currency that falls from rift bosses and trades freely, so a friend can hand you the essence you are short. Take the band off before you bring it to her: she works on what is in your bags, and she does nothing at all unless you are standing at her forge.
The tracker on your screen
While you are inside, a small strip on your screen keeps you oriented: which floor you are on out of how many, and a live countdown. Read that countdown carefully, because it is not your run running out. It is the entrance back in the world closing. Once you are through, your group plays the rift out at its own pace, however long that takes, but when that clock reaches zero the way in is gone for everybody, so think twice about stepping outside near the end of it.