A Daedalus Hammer is an Artifact that improves your weapon by upgrading its attacks. It can appear in the first room of a run, as a room-clear reward, or can be purchased from Charon for 200 before the Temple of Styx. Only two Hammers can be acquired during any given run, and their effects will stack, although buying an Anvil of Fates from Charon in the Temple of Styx can allow the player to have three Hammer effects at once. Only one hammer can be found in Tartarus or Asphodel, with the second appearing in Elysium. If fewer than two hammers have been acquired upon entering the Temple of Styx, it is possible for one to be found there. Although up to two Hammers will always be offered, it is not guaranteed they will be offered again, should Zagreus choose to not take it. Some upgrades may be mutually exclusive with others. The effects of the hammer are reset upon Zagreus's death.
The Hammer's upgrades vary from weapon to weapon, with some being minor adjustments and others shifting the playstyle of the weapon entirely. All upgrades affect either the Attack or Special of a given weapon; Zagreus's Cast is unaffected.
More information on how the hammer alters the weapons can be found on their respective pages: