How do I prevent a teleporting artefact from splitting the party?

Question

During my campaign the players will find a magical diary. The diary is an artefact that, when read, transports the reader into the time period described therein. They can interact with the people and events described as though they were there at the time.

(The reader can take other people with them by reading the diary aloud. This is important because most of my party are illiterate.)

Some worldbuilding background can be found here.

To keep the game running smoothly, I am looking for advice to stop the artefact splitting the party. For example half the party is inside the alternate reality, and the other half is somewhere else.

One concern is that the Wizard -- the only character who can read -- might examine the book at the end of the day while the other characters are sleeping, and end up inside it. Then I have to run a solo adventure with the Wizard until they find their way out. This is no fun for the other players.

Another concern is the party might be split in different parts of the same town when they find the book. The half that finds the book examines it and ends up inside, and the other half is still in the town.

Later in the campaign, once the players figure out the artefact opens a portal to somewhere else -- wherever that is -- I expect them to only go as a group for safety. Otherwise I may tell them in between games, as an addendum to session 0, that I prefer if they go into the book together. Then there is no problem splitting the party.

But what about earlier on -- how do we avoid splitting when the book is still mysterious?

I can think of two approaches. The first is changing the rules for entering and exiting the diary. One idea is to make it easier for extra players to join the reader, for example by touching their body they get sucked in with them.

The second concerns exactly when and how the players find the diary, and how much info they have. For example foreshadowing the diary as a powerful and dangerous object, so the players will be wary to examine it on their own.

Does anyone have advice on how to avoid splitting the party?

Answer 1

Have the artifact teleport the whole party

Artifacts are foremost plot devices, but in 5e, they can be identified as easily as any old magic item. That makes it very difficult to have the artifact be mysterious if one of your players can cast identify, or even just handle it for a short rest. You'd need to build special provisions into the artifact that these means won't work. If you do not, you may not even have the problem, as your PCs might easily learn about the risks of reading the book before doing so. But you never know, the wizard's PC might as well decide reading it before ever examining it.

Getting around the party splitting is much easier:

Artifacts can do whatever you want them to do. Just define that the person that is teleported by activating the artifact is teleported along with all their closest associates (no matter where these are at the time).

Answer 2

Put a lock on the book that requires multiple people to be present.

For example, the book might require X number of people to be touching the book. You could signify this with X small gems on the book, and for each person touching the book, one additional gem lights up. Should be pretty easy to figure out by communicating to your wizard (or whoever touches the book first) that whenever he touches it, one of X gems lights up.

You can also go a darker route by requiring the blood of X different people for opening the book, but it goes without saying that is likely to deter your players and make them wary. As such, you should only do something like this if the book is indeed somewhat sinister.

Either way, these restrictions should only apply for opening the book. Once it's opened, I would expect that the party actively wants to stay together until they find out what the mysterious book is about.

Require multiple abilities for opening the book that force multiple, if not all, party members to be present.

As you likely don't know which way the party will split, you should prepare conditions for all party members. You can then decide on the fly which ones are required, choosing in a way that forces the party together.

Examples of such abilities could be picking a lock (rogue), requiring a strength check to force something open (barbarian/fighter), reading something in a certain language (applies to all party members with a language that only they are proficient in), opening a seal that only opens when a follower of a certain god(dess) touches it (cleric/paladin), or saying something in Druidic to cause vines to disengage Harry Potter Parseltongue-style (druid).