Lethal Poison

Potion effect that subtly drains your health at a random interval.
It doesn't hurt you, instead, it subtracts your health directly, making it much scarier and able to bypass the usual 0.5 seconds of invulnerability after taking damage.

The effect is reversed on undead mobs. (Heal instead of hurt)

The exact formula goes like this:
The amount you get hurt is based on the level of the potion. (1 damage for level I, 2 for level II...)
At level I, 1/30 chance to be hurt every tick. On average, you are hurt once every 1.5 seconds.
At level II, 1/25 chance every tick.
At III, 1/20 chance. You get hurt once per second.
At IV, 1/15 chance.
At V, 1/10 chance. On average, hurt twice per second. The sound effect stops playing at this point.
At VI, 1/5 chance. Hurt 4 times per second on average.
At VII and above, you're hurt every tick.

Normal: Lethal Poison I for 0:45
Strong (Glowstone Dust): Lethal Poison II for 0:21
Long (Redstone Dust): Lethal Poison I for 1:30
Brewing ingredient: Golden Spider Eye

Lethal Poison can also be brewed from inverting a Healthy potion with a Spider Eye.