Basilisk

The Basilisk turns you to stone slowly, which is the whole point

How to run it. 52 Hit Points, AC 15, Speed 20 feet, Initiative −1. Its Bite at +5 does 2d6 + 3 Piercing plus 2d6 Poison. That is a CR 3 animal. The danger is the Bonus Action.

The rule that matters. Petrifying Gaze, Recharge 4–6, so it comes back on half its turns. Everyone in a 30-foot Cone makes a DC 12 Constitution save. A first failure only Restrains. The target saves again at the end of the basilisk's next turn, and a second failure turns it to stone. Two failures in a row:

The save covers every creature in the Cone; the stat block asks nothing about looking at it. So shutting your eyes does not save you here.

The mirror. If the basilisk sees its own reflection in the Cone, it has to make the same save. With its Con +2 it fails 45% of the time and ends up stone 20% of the time. A polished shield held up in the Cone is a real plan, not a joke.

The common mistake is rushing the second save. That round between "your legs are going grey and stiff" and the roll is the best scene the monster offers. Let the party spend it: someone kills the basilisk, someone drags the friend back, someone prays.

For players. Kill it fast. It is slow and fragile compared to its threat, and every turn it lives is another 50% chance of a fresh Cone. Spread out so one Cone can't catch three of you.

The human angle. Humans are wired for gaze. Atsushi Senju and Mark Johnson's 2009 review calls it the eye contact effect: being looked at changes how the brain handles whatever comes next. A monster whose weapon is its stare hits something older than the rules.

Source: Senju, A., & Johnson, M. H. (2009). The eye contact effect: mechanisms and development. Trends in Cognitive Sciences, 13(3), 127–134. doi:10.1016/j.tics.2008.11.009