Endless is a mode that was added with the installment of Slayer Edition. Unlike in Campaign, Endless has no plot line to follow, but rather provides an infinite number of procedurally generated floors for players to complete and play indefinitely. Each of the 3 realms from Campaign are accounted for in Endless, and their order follow a distinct pattern: 6 Crypt floors, 6 Cave floors, and 6 Lava floors (the Cave and Lava floors could be further broken down as Caves are 3 floors of nests and 3 of mines, while the Lava floors consist of 3 floors of temple and 3 of volcano)—making a total of 18 floors before the cycle repeats. On the last floor of each realm (every 6 levels), Death Incarnate will make an appearance.
Resuming your progress in Endless is done quite unconventionally. Portals are unlocked after every 18 floors, and to resume your progress at a later time, you must start at floor 1 and use one portal to reach the next portal floor until you reach the deepest floor available to you.
Unlocking Portals with Host
It is possible for both host and guest players to unlock portals together but only under particular conditions. All players that intend to share in the host's portal unlocks must have the same highest portal as the host. For example, if the host's highest portal is on floor 55, any guest player that would unlock a portal with the host must almost have their highest portal on floor 55. In addition, any such guests must be present starting from the character selection screen; any guests that join after the character selection screen will not be eligible to unlock any subsequent portals within that session.
Causing Portals to Not Save
Players sometimes use the "Drop In" and "Drop Out" options for various reasons (such as changing characters), however, be warned. If player 1 (the player is on the far left, if there are multiple players) uses the "Drop Out" option at any time, it will cause any newly reached portals to not save from that point forward during that session. The game will not warn you about this in any way; it will simply not save any further progress so take caution.
In Endless, the difficulty does appear to continue to increase even past floor 37 (by way of increasing monster count as opposed to difficulty modes), but only up to a certain point. Beyond this point, the floor numbers simply increase while the floors themselves cease to necessarily become harder (for example, floor 20,000 is not necessarily more difficult than 200).
At the start of each floor (except floors containing portals), you will be given an opportunity to buy up to four items, such as food, potions or Skull Coins. As to which of these items will be available for purchase on any given floor is random.
Although potions and Skull Coins cost the same amount of gold, potions are usually considered to be more valuable as they can often be used to generate coins or save multiple lives.