Ultra Hardcore is amode of Minecraft that closely resembles the Hunger Games. Players are dropped randomly in a bounded landscape with nothing. From there you make scanvenge, build tools, and hunt down other players Battle Royal style.
The two major modifications to Vanilla Minecraft are firstly we're disabling the night/day cycle and secondly disabling health regeneration. The former to allow for more player interaction and less deaths from aggressive mobs. The latter so every point of damage is permanent, so running away to regen health is not viable, and small things, like fall damage, must be accounted for.
This primer will cover the basics, new additions to the game that are useful, PvP advice, and some more advance tactics yourself and other players may use.