Someone probably already said this considering how obvious it seems but is it the Reginator? The Reginator is an ancient being that stays still like a statue, hence shall stay dead. The Reginator also has control of the three Regi brothers and is strength-wise, the strongest Pokemon in existence as seen in Pokemon Adventures where it overpowered powerful legendary Pokemon quite easily.
However I have a second guess. If it's not the Reginator, is it Giratina? At first the riddle sounds like Arceus but it kind of makes this powerful being sound like it isn't quite at the top which is not Arceus. The ancient thing that shall stay dead could refer to Giratina's ghost-type and his eternal imprisonment of the Distortion World where he is both the ruler and its prisoner. He has the power to challenge Dialga, Palkia, the two above, and to an extant Arceus, the swift end of the three but Arceus is the most difficult one to challenge, hence can't end one so easily. Not only that but although Giratina is seen as the strongest of the three, he cannot bring life like Arceus.
Not only that but Giratina is interpreted as a vengeful Pokemon that holds a grudge and if he were to be freed from his world, he might wreak havoc on the Pokemon world as revenge against Arceus, hence Arceus end might Be Giratina's fun. In another interpretation of Giratina, he shares his world with Cyrus in Pokemon Generations.