Rod of the Pact Keeper

From Dungeon Master's Guide, page 197. Found on Magic Item Tables F, G, and H.

Description
Rod, uncommon (+1), rare (+2), very rare (+3) (requires attunement by a warlock)

While holding this rod, you gain a bonus to spell attack rolls and to the saving throw DCs of your warlock spells. The bonus is determined by the rod's rarity.

In addition, you can regain 1 warlock spell slot as an action while holding the rod. You can't use this property again until you finish a long rest.