Gruz Mother

Gruz mother is an optional mini-boss in Hollow Knight. She is also a dual Colosseum of Fools boss. Unlocking her Journal entry requires beating 3 Gruz Mothers.

In-game events One Gruz Mother can be found blocking the direct path to an abandoned village at the bottom of the Forgotten Crossroads. She is sleeping, exhausted from carrying her children, and wakes up when struck. Upon defeat, her stomach bursts open to reveal a swarm of Gruzzers.

Two more Gruz Mothers can be fought simultaneously at the end of the Trial of the Warrior. They do not spawn Gruzzers upon death.

Behaviour and Tactics Gruz Mother is found sleeping in a small arena. The battle begins once she is attacked.
 * Charge: Gruz Mother charges a short distance straight at the Knight's location, and then either stops or lightly collides off the floor or a wall.
 * Wild Slam: Gruz Mother repeatedly slams herself between the floor and the ceiling slowly moving a short distance forwards with every slam and turning without stopping when reaching a wall.
 * Fly: While not performing any attack, Gruz Mother aimlessly flies within the arena.

When Gruz Mother dies in her Forgotten Crossroads encounter, she spawns a swarm of 7-8 Gruzzers that must also be defeated before the Knight can leave the boss area.

The Wild Slam attack can be dodged more easily with the Mothwing Cloak since the attack is fast and decent timing is required to avoid it otherwise. Using the Vengeful Spirit Spell is the easiest way to defeat her, as she has relatively low health (6 Vengeful Spirits/18 Old Nail strikes is enough to defeat Gruz Mother).

In the Colosseum of Fools, two are fought at once. It is best to kill one Gruz Mother as fast as possible, and using all SOUL for Spells is a good idea since the next wave may be Zote the Mighty, who cannot deal any damage to the Knight. Howling Wraiths/Abyss Shriek is the best Spell to defeat Gruz Mother in the Colosseum of Fools. Otherwise, use the Fragile Strength/ Unbreakable Strength or Quick Slash Charms to kill her quickly. Healing is easy since they do not attack very often, just keep a small distance from them when healing.

Location Gruz Mother can be found in the Forgotten Crossroads, sleeping peacefully until disturbed.

Two are fought at once in the Colosseum of Fools, however, they do not spawn Gruzzers upon death. This is also true for the ones fought in the Hall of Gods,  Pantheon of the Master, and  Pantheon of Hallownest.

Achievements

Trivia
 * Gruz Mother was the first mini-boss revealed in a YouTube gameplay demonstration video. It was referred to as "Empress Muzznik".
 * If the Knight defeats the Gruz Mother blocking access to the deserted village but dies to the swarm of Gruzzers, the boss is still counted as dead and the Knight can access the village.

References

Матка Жужж Майка Грузер Madre_Gruz Grummermutter Cicalina Madre Matka Gruzzerów Mère Gruz