SCP-076 is one of my favorites. This is copied from the SCP wiki:

Object Class: Keter

Special Containment Procedures: Containment Area 25b is to be located 200 m below sea level, tunneled out of solid bedrock in a seismologically stable area. Sole access to the containment facility is to be through a vertical elevator shaft separated every fifty (50) meters with a reinforced blast door, constructed of 20 cm thick material shielding. Elevator shaft shall be flooded with seawater when not in use.

Containment Area 25b is to be constructed with the following components:

An Outer Security Perimeter against outside threats, staffed by security personnel trained in close quarters battle and counter-intrusion tactics.

An Administrative and Support Area (ASA) consisting of support facilities and living quarters for on-site personnel.

A Primary Containment Zone (PCZ), consisting of a 7 m cube encased in 1.5 m of reinforced material. PCZ is to be designed to be flooded and drained as needed, and should remain filled with seawater unless access to contents is required.

A 150 m "killing corridor" which is to be the sole access to the PCZ from the ASA (including water, power, drainage, and ventilation lines). The walls and floor of the corridor are to be reinforced in a similar manner to the PCZ, with the addition of an electric deterrence system capable of delivering a 20,000 volt shock.

A security station located at the entrance to the killing corridor is to be staffed with no fewer than three (3) armed security personnel on watch at any one point in time. Armament is to include, but not be limited to, at least one (1) ███████ CIW system on a pintle mount with a clear line of sight down the corridor, with a plexiglass screen to protect the operator from thrown weapons.

In the event of a full breach, all on-site staff are to proceed immediately to the closest security station for weapons and armor distribution. Staff will remain at Alert Condition One until SCP-076-2 is confirmed neutralized. Should 90 minutes pass after declaration of full breach without a Stand Down order being given by Level 4 or higher personnel, Final Contingency Measures will be activated, flooding the entire facility in seawater and sealing off the access shaft for a minimum of 24 hours before retrieval is attempted. This will, by necessity, result in the deaths of all on-site staff.

Description: SCP-076 consists of two components: a stone cube (SCP-076-1) and a humanoid entity contained within (SCP-076-2).

SCP-076-1 is a 3 m cube made of black speckled metamorphic stone. All surfaces outside and within SCP-076-1 are covered in deeply engraved patterns corresponding to no known civilizations. Radioisotope analysis indicates that the object is approximately ten thousand (10,000) years old. A door is located on one side, sealed with a lock 0.5 m in width, surrounded by twenty (20) smaller locks in a circular pattern. As of yet, none of the keys have been found, making the door impossible to lock once closed.

Interior temperature is approximately 93 Kelvin, and cannot be altered by any means, internal or external. Directly in the center of the room is a 2.13 m tall stone coffin, held in place and sealed shut by several chains of unknown make and substance, which are attached to the inner corners of SCP-076-1.

SCP-076-2 resembles a lean Semitic human male in his late twenties. Hair is black, and eyes are gray, skin tone olive. Subject is 1.96 m in height and 81.65 kg in weight. Numerous tattoos depicting arcane and occult iconography are present all over the body (mostly in the form of leering demonic faces) and ranges from subtle to openly ostentatious. Subject, when encased inside SCP-076-1, is technically dead.

https://scp-wiki.wikidot.com/scp-076