Storm King's Thunder
A companion is an NPC that travels and fights alongside the player characters, but takes a less important role than them. A companion has a full set of statistics, as normal for any creature, but most of the time, those statistics won’t matter. Companions don’t move on their own, make attacks, cast spells, or use their other special abilities, but they also can’t be attacked, take damage, or be affected by most abilities (but see Injuries below), and they provide one or more special benefits, determined by the DM, to the character they travel and fight besides.
Assignment: A companion must be assigned to a character in order to have a mechanical effect. An unassigned companion still travels with the party and will still participate in conversations, but does not participate in combat, or most other challenges.
Assigning a companion is usually a matter of making the decision during a rest or while taking a moment to plan before proceeding. In combat or any other tense situation, you can freely assign any willing companion to yourself during your turn that is within 5 feet of you, including changing the assignment of that companion from another character to yourself, but such a companion provides no benefit to you until the start of your next turn.
Companion Benefits: While a companion is assigned to you, it moves with you and occupies your space, and you have all of the special companion abilities, or in some cases drawbacks, it provides.
Typically, a companion grants you three abilities: one you can always use, one the companion must spend inspiration to activate, and one you have access to only if the companion is loyal. Individual companions may vary, but this is the standard.
Unless otherwise noted, a companion can only use one of their abilities that does not have a triggering condition during your turn, and cannot use such an ability outside of your turn. Triggered abilities can be used any time they apply.
Some companion abilities might be usable one or more times per encounter. Rather than recharging during a rest, these abilities recharge any time you enter a new “scene.”
Inspiration: Many companions can gain inspiration just like PCs can, but rather than gain it through roleplaying their own traits, flaws, etc., they gain it from speaking with the PCs. A companion gains inspiration after any meaningful conversation with a PC, whenever a PC helps the companion accomplish a significant personal goal, or any other time the DM feels is appropriate.
Companions also don’t spend inspiration normally, since they don’t make rolls. Instead, a companion that can gain inspiration will have at least one ability that will benefit in some way from spending inspiration. Usually, this simply means it costs inspiration to activate.
Loyalty: Companions can become loyal, perhaps over time, or after the PCs complete a quest the companion has a personal stake in. Companions that can become loyal provide some additional benefit while they are loyal. This is typically an additional ability, but it might be an upgrade to an existing ability, multiple new abilities, etc.
If you are using the loyalty system detailed in the DMG (p. 93), a companion is loyal if their loyalty score is 10 or higher. Otherwise, the DM determines when they are considered loyal, such as after you complete a quest with personal significance to that companion.
Age: Some companions, such as baby dragons and other monsters, have age boxes, which are filled as they grow up. This usually replaces loyalty, but a companion could have both. For every age box checked, the companion gains another injury box, and like loyalty, some abilities may require a certain number of age boxes to be checked.
Injury: While companions can’t be attacked, and their hit points totals are irrelevant, they can be injured. A companion has a number of injury boxes, which abstractly represent their ability to resist damage. Each time one of the following circumstances occurs, each of your companions take an injury. You can spend your inspiration, or that companion’s inspiration, to allow one of your companions to avoid taking an injury.
You take at least 10 points of damage from an area effect. The amount of damage required increases to 15 at 5th level, 20 at 11th level, and 25 at 17th level.
You take a critical hit
You drop to 0 hit points
You take damage while you are at 0 hit points. You do not suffer a failed death saving throw from this damage if you have at least one companion with unchecked injury boxes.
If all of a companion’s injury boxes are filled, they become unassigned and begin dying as if at 0 hit points. A companion can be stabilized as normal, and if they regain any hit points, they regain consciousness, but their injuries are not necessarily healed (see below). A stable companion regains consciousness if any of their injuries are removed.
A companion cannot be assigned if all of their injury boxes are filled, but they can travel and speak normally if they have regained consciousness.
An average companion has 3 injury boxes, but the number may vary. A companion heals one injury, unchecking the box, after each short rest, and heals all of them after a long rest.
Healing magic, alchemical healing, or other healing effects, besides hit dice, can also heal a companion’s injuries. A companion unchecks a single injury box for every two dice worth of healing that is applied to it. Every 5 hit points worth of diceless healing counts as one die: 10 counts as two dice, as does 1d8+5, etc.
Companions to the Rescue: During your turn, if you take no action and do not move on your own, your companions can take the Use an Object action. Alternatively, they can move you up to half your speed if they are capable of carrying you, or 5 feet if they must drag you. This is especially helpful if you are stunned, bleeding to death, have been turned to stone, etc.