Deadman mode is a gameplay mode where almost everywhere is PvP. Players of all combat levels are allowed to fight eachother, but when they attack another player they become skulled (for longer than normal). Skulled players are actively targeted by gaurds (beefed up of course) in established areas all over Runescape. The game currently has a 5x normal XP gain rate, but if you are killed you lose 50% of your XP in all skills EXCEPT for 5 of your choosing (2 combat only). The details aren't super clear yet, but it sounds like an intense, thrilling new way to play Runescape.