Blind - Your screen is fogged up. Monsters that are blinded have trouble firing projectiles accurately at you.
Madness - You see illusions of other monsters. Monsters that are under the influence of madness will randomly attack the air around them.
Haunted - A phantom appears that attacks you. Against monsters it is essentially the same - a phantom appears that attacks them. If an enemy is haunted the phantom is blue in color to distinguish it.
Unstable - You move slower, and when damaged become either frozen, poisoned, on fire, or shocked. The status basically functions the same for monsters.
Doom - A countdown starts that, if it reaches 0, causes your knight to lose all his hearts. Doom can be removed by remedy pills, by defeating the monster that applied Doom, or other special circumstances. Monster-only status effect.
Edited the OP: Haunted is a different status from Doom now.
Doom - A Grim Reaper appears and hunts you. If he hits you, you die. Monster-only status effect.
"Red Elf needs food badly."