Ultra Hardcore, or UHC, is a popular Minecraft minigame that can also be a gamemode in normal Minecraft. In UHC, there is no natural regeneration, which makes it especially hard.
UHC is a rare minigame that can be played between you and your friends on a local LAN game instead of having to find a server that hosts it. In order to play Ultra Hardcore, you first need to start a game with cheats enabled. Then you need to type /gamerule naturalregeneration false into the chat box. To set a border, type /worldborder (x) (z) into the chat box, replacing x and z with the x coordinates and z coordinates of the border.
On servers, the concept is similar to in vanilla, but with a few differences. Natural health regeneration is turned off, and after one death you are eliminated, like most survival minigames. There are two stages, the first for gathering resources, and the second as a deathmatch. UHC games can last extremely long, with times ranging from 15 minutes to 2 hours. In some versions the border shrinks, in others it does not.