I wan to make some warlock specific magic items like the ones for sorcerers and wizards in Tasha's. I want them to be themed after ancient artifacts, and I want some ideas for what they could do.

To be honest, some of the ones in Tasha’s were “borrowed” from already existing Warlock specific Magic items such as the Rod of the Pact Keeper.

I want them to have more specific benefits than just a + something to attack and save DC. Something like the sorcerer shards, but in the form of artifacts (not the rarity) left by the different types of patrons.

Ahh. But those shards all trigger “when you use a Metamagic option on a spell,” so what would be the trigger for these Warlock items?

Maybe "When you cast a Warlock spell of 1st level or higher," so that the use is still rather limited?

what about rod of the pact keeper?