Dungeon Master’s Guide, pg. 197
Rod
Rare requires attunement by a warlock
While holding this rod, you gain a +2 bonus to spell attack rolls and to the saving throw DCs of your warlock spells. 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.
Cost: 4,000 gp