Trial Parties and Instances

Who Comes With You

Making the offering starts a 30-second count. A big floating counter over the altar shows the trial's name and the seconds left: white, then gold from 10 seconds, then red and larger for the last 5. You see "<trial> opens in 30 seconds. Everyone within 10 blocks of the altar when the count ends goes in with you." Players nearby are told "<trial> is opening here. Stand within 10 blocks of the altar when the count ends to go in.", and each player's action bar shows whether they're inside the circle ("entering in <n> s") or not ("step into the shrine's circle to join").
When the count ends, the party is everyone within 10 blocks of the altar, squad or not, who isn't already in another trial. Anyone outside the circle is left behind. A player still on their 15-minute cooldown for this trial is left behind and told how long they must wait.
Only the leader pays and only the leader's level is checked. Everyone else comes along free, whatever their level.
Everyone in the party gets a dark purple action-bar message, "The trial takes shape: <trial name>...", while the battleground is built, and their return point (the altar) is saved.
No one can join a trial once it has started.

The Trials Dimension

Trials take place in their own dimension, the Trials dimension: an empty void world with no terrain, always at noon, with sky light. Beds and respawn anchors don't work there, and there are no raids.

Every running trial is its own private instance, so any number of parties can run the same trial at once.

Entering a Trial

When a trial starts, its battleground is built fresh: even the biggest, the Nine-Tails' 170 x 70 x 170 valley, takes about two and a half seconds, and most take well under one.
Once it is built, a shell of barrier blocks is put round all six sides of the battleground, one block outside it.
The party is teleported in together, standing side by side 1.2 blocks apart at the arrival point, facing the boss's position.
Each player sees a title: the trial's name in gold, with the subtitle "Survive the waves, then face the master" (arena) or "Clear every room to break the seals" (dungeon). An End-portal sound plays.

Rules Inside

Blocks cannot be broken or placed (except by players in creative mode).
Explosions break no blocks, though they still do damage.
Logging in inside the Trials dimension when your trial is no longer running sends you back to your shrine (or, if it can't be found, the world spawn).

Who Counts As Still in the Trial

A party member stays in only while they are online, alive, in the Trials dimension and within 64 blocks of the battleground.
Whoever dies, logs out or leaves any other way is out of the trial for good and loses any reward (they aren't put on the 15-minute wait for it either). A player who dies respawns normally (bed or world spawn), not back in the trial.
When no one is left, the trial ends with no rewards and its place is freed for the next party.

Stray Pull-Back

Any party member who strays more than 2 blocks outside the battleground, for example by falling off an edge or flying out, is instantly teleported back to the party's anchor point, with no fall damage. The anchor starts at the arrival point and, in a dungeon, moves to wherever a party member first stepped into each new room.
Goons that fall more than 8 blocks below the battleground's floor are removed and counted as beaten.