"untold power" is a correct and usable phrase in written English.
It can be used to describe something that has immense or limitless power that has not yet been revealed or fully understood. Example: The ancient artifact was said to possess untold power, capable of granting its wielder unimaginable abilities.
Whoever owns the magic bottle will gain untold power.
It isn't – but it gives them untold power over the minds and insecurities of female stars.
It will give them untold power if they find it, so Indy must stop them.
They deploy untold power and enjoy total obedience as commanders-in-chief, but in the civilian domain are checked and balanced at every turn by a fractious polity.
The Andromeda strain of space germs mutates into harmlessness and goes away; the lost city of the Congo is wiped from the map by lava; in "Sphere," the discoverers of the extraterrestrial artifact of untold power use that power to wish it into retroactive nonexistence.
When this state of mastery is achieved, untold power, intelligence, and wisdom become accessible to us, like magic.
Nobody knows what happened to the Stone – if it really existed – but there are all sorts of myths and theories about where it came from (some people have even suggested it could have been an alien spaceship!) and the untold powers that it might possess.
In March 1942, Nazi officer Johann Schmidt and his men enter the town of Tønsberg in German-occupied Norway, to steal a mysterious device called the Tesseract, which possesses untold powers.
And now, suddenly, we realise that the US government has a huge secret stash of similar objects with untold supernatural powers.
Over the course of his rise to power and untold wealth, the diminutive mathematician and onetime member of the Russian Academy of Sciences helped form the two key institutions that have defined modern Russia and, arguably, continue to hold it back.
To hear Mr. Suharto's name read out in a criminal case was a momentous event in a country where for three decades he was synonymous with untold wealth and unassailable power.
