The Rod of the Pact Keeper is another magical item from D&D that doesn't have any official art. I think that might be because there are so many Oaths that Warlocks can make and varied enough deities they can make those Oaths to that there would have to be dozens of different Rods.
I like to think this version would fit a Warlock that takes the Pact of the Undying.
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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.