The Queen Alien.
In the Alien franchise, queens are the matriarchs of the Xenomorph race, responsible for propagating the species. They are considerably larger than the common xeno soldiers, standing over ten feet tall, with distinct physical attributes unique to their caste. Xenomorph hives are dependent on queens to survive and expand since they are the only caste members that can reproduce.
Physiology
Queens look very different from standard xenomorph soldiers: they stand at three times the size, have four arms and a large ornate-looking crest on the backs of their heads. Their upper arms end in six digits and their smaller arms have three; these secondary arms are believed to be used for handling eggs inside the nest. The queen's face is capable of withdrawing into the armoured carapace of its head, much like how a turtle withdraws into its shell when threatened.
For most of its life, a xenomorph queen remains inside the centre of its hive and continuously lays eggs. The queen's egg sac is tremendous in size, larger than the rest of her body, and can contain hundreds of eggs at any given time. A queen is immobile while in her egg-laying phase and will lay dozens of eggs every hour. Because of this immobility, the queen is forced to rely on drones to tend to her feeding and praetorians for protection. If the need arises, such as a disaster that will destroy the entire hive, a queen can free herself from her giant egg sac and discard it in order to become mobile and flee to safety. Severing the egg sac is very painful for the queen, though queens can apparently regrow these organs after severing them.
Behaviour
The queen's egg sac
Under ideal circumstances, xenomorph queens remain in their hives indefinitely, only moving around if the hive is in danger. They remain immobile until their egg sacs need to be shed, and until such time their needs are met by a contingent of worker drones that feed her and protect her.
When a queen discards her egg sac, then that means she and/or the hive is in danger of destruction. The queen is very capable of dealing with threats herself, using her tremendous size and brute strength to her advantage. A queen will fight in much the same way as her underlings, using her claws, fangs, tongue and tail to attack. Given her greater size, a queen has a greater attack range since her arms and tail are much longer. The queen's carapace is much tougher than other xenos and equally as likely to spray the xenomorph's acidic blood when breached.