Morale

By default in D&D 5th edition, all creatures fight until the death and there is no system for evaluating scenarios which might cause a group of creatures to flee. But this doesn't really make sense to me. High intelligence creatures have a sense of value on their life and will likely decide to flee if the chance of victory is lower than the chance of dying, and lower intelligence creatures will have a natural fight or flight response which may lead them to flee a combat. In addition to the narrative value of morale checks, it greatly accelerates combat and trims the length of the "slog" phase where it just becomes an HP grind and the party knows they are going to win. It also creates an interesting dynamic with the possibilities of fleeing creatures: do they warn other creatures, do they fall back to a fortified position, does the party decide to give chase to prevent them from performing these advantageous actions? In my eyes, these are all much more interesting scenarios than repeatedly grinding HP to 0.
All enemies in a combat encounter start with a morale score, which is the critical value which will cause them to flee should they roll it on a morale check (higher morale scores are better). In a campaign focused on horror, threat, and less on heroism, this system may optionally apply to the players as well, but generally 5th edition is designed around the premise that players are heroes, so I tend to avoid imposing systems which detract from their agency.
The starting morale of a creature is determined based on the nature of the creatures, their experience, and their motive for fighting. There is not a strict calculation, but it is more of an abstract decision made by the GM based on the narrative situation. A set of creatures may share a morale score, or they may have independent morale scores. The different ranks are:

/* Combat - Morale Levels */

The morale level can change during the course of an encounter. A morale check is determined by rolling 2d6. If the result of the roll meets or exceeds the current morale rating of the group of creatures, they will cease fighting. This often means fleeing, but could also mean surrendering, or attempting negotiation, especially if they are pursued when fleeing. There are number of situations which can alter the morale of an enemy, including but not limited to:

Having an ally slain: For intelligent creatures, this may give a bonus to morale as they now have additional motive to fight for revenge.

Having your leader slain: This will instantly break morale for minions. For non-minion creatures, this will impose a -5 to morale and instantly trigger a morale check.

Having an ally turned against you, often through magic: This will impose a -3 to morale.

Magic: Non-magical creatures will panic when magic is used against them imposing a -1 or -2 penalty to morale. Magical creatures, especially those with magic resistance, will not falter under threat of magic.

Falling below 75% HP: -2 penalty to morale and instantly triggers a morale check.

Falling below 50% HP: -4 penalty to morale and instantly triggers a morale check.

Falling below 25% HP: -6 penalty to morale and instantly triggers a morale check.

Having 25% of your allies slain or fleeing: -2 penalty to morale and instantly triggers a morale check.

Having 50% of your allies slain or fleeing: -4 penalty to morale and instantly triggers a morale check.

Having 75% of your allies slain or fleeing: -6 penalty to morale and instantly triggers a morale check.

Morale checks are also performed at the start of an encounter, or before determining if enemies will pursue the players should they decide to flee. A character can attempt to force a morale check as an action by making an intimidation check against a target, with a DC equal to its current morale level. Enemies additionally gain progressive levels of fear if their morale decreases below 8:

6: Shaken

4: Frightened

2: Panicked

This will force an enemy to flee should their morale drop low enough.