Unbreakable Vow
Incantation: None. It manifests itself as interlinking chains of fire binding the clasped hands of the people taking the Vow; the fire shoots out as a tongue of flame from the wand of the Binder (a witness to the Vow) every time the person who takes the vow makes a promise. The flames then form into the linking chains.
Description: Causes a vow taken by a witch or wizard to be unbreakable, but I'm sure you've worked that part out yourself, funnily enough (If you know where that is from, virtual high five!). If he or she should break it, the consequence is death.
Seen: Snape takes an Unbreakable Vow with Narcissa Malfoy at the beginning of HBP, vowing to help Narcissa's son Draco with a task given to him by Voldemort, and to finish the task should Draco prove incapable. Fred and George attempted to force an Unbreakable Vow upon Ron as children.
Unbreakable Charm
Incantation: Unknown
Description: Causes an object to become unbreakable.
Seen: Hermione uses this spell in GoF on a glass jar containing Rita Skeeter in her unregistered animagus form (a beetle) so as to make sure she could not return to human form.
