What part of Hexblade Warlock allows Tiangong to absorb the form/powers of a magical weapon?

Nova took Pact of the Blade when she got the Pact Boon feature from her 3rd level in warlock. It lets her create almost any weapon she chooses whenever she summons it. I don’t know if she took the Improved Pact Weapon as an Invocation (I assume she did) but that lets her use it as an arcane focus for her spells and in combination with her Hex Warrior feature she can use her Charisma mod instead of Str or Dex. That’s why she can transform Tiangong into any weapon, as for the absorbing properties of another magical weapon I think that’s just some homebrew that Mark and Kim came up with to set Tiangong apart from other magical weapons and establish it further as a magical being that can qualify as a patron.

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