The what
Multiboxing refers to the playing of multiple separate characters concurrently in an MMORPG.
The why
Initium's no place for the storybook hero. It's a world of endless struggle and cold chance. In the terms of the game, characters cap early, and they're never safe from being one-shot. In the terms of what the hell does that mean, a violent death awaits even the mightiest of warriors.
To reduce your odds of being cut down while you run like the coward Initium reveals you to be, or to at least ensure that when you do inevitably die, all the progress you've been lugging around on your person doesn't vanish like the life from your pleading eyes, you'll want to roll around in a party of your own control. In other words, you'll be expanding laterally into other characters long after you've hit the ceiling on your first one. That's how Initium is played, and it goes much smoother when you can do everything at once on a single screen. The means for that aren't builty directly into the game however, hence this guide.
For more information on progressing into additional characters, see Vocandin's Alt Making Guide.
Setting up in-game
You'll need a different account for each character you want to play at the same time. That's just how the game works, even if you're using more than one device: when you do something with one character, you'll be picking up from there in any other instance of the game, as refreshing the page will make clear. You may not notice at first, if the game allows you to issue a command for the character you've still got on the page and then resume with that character. But you won't truly be playing both simultaneously.
Remember the maximum party size of 4. Also consider buying premium, which will allow you to self-revive.
Browser profiles
So you've got two or more accounts. You might expect multiboxing to be as simple as logging onto each of them in different browser windows or tabs. But much like with different characters on the same account, each tab will pick up where you left off from the last—that is, unless they're using different sets of cookies.
A simple way to manage that is to use additional browsers and/or private browsing tabs. Chrome along with its incognito feature plus Firefox and its private browsing will allow you to play 4 accounts at once. But that is a very inelegant (if commonly used) solution. There are better ways.
My tool of choice is SessionBox. It has a variety of useful features, but we don't need much for our purposes. Install it here, ideally for Firefox. It should place a box icon to the upper right in your toolbar. Click on the icon, log in as a guest, and begin as many new sessions as you'll need. You should get a tab for each one. Use them to log into your different Initium accounts. Notice the new sessions don't yet have any login info saved.
Tiling
Tile Tabs is a free Firefox add-on that will allow you to set up your tabs so they're all visible at once. Install it, then find its icon in your toolbar. Select the left-most tab you want to tile and ensure the others follow immediately to the right. Then click the down-arrow by the icon and select New Layout followed by an arrangement of your choice. You should end up with something like the example I provided in the screenshot toward the beginning of this guide.
The ability to tile tabs is the reason I prefer Firefox over Chrome for Initium, by the way. As far as I know, Chrome currently lacks that functionality. A version of Tile Tabs, and other programs like it, do exist for Chrome, but they only allow the tiling of windows. Anyone using Windows 10 can also tile windows by pressing any arrow key while holding down the Windows key.
Notes and a warning
And that about sums it up. You should have different accounts in different tabs under different browser profiles tiled across your screen. Feel free to contact me with questions or suggestions.
Finally, everyone should be aware that rapidly issuing commands from several different characters may either flag you for bot checks, or increase the frequency with which you're prompted with checks if you're already flagged. There's nothing in the rules about multiboxing however.