The term ancient powers is used imprecisely in common speech, applied to anything old and threatening. In formal research it refers to a specific category: forces or entities that predate the Elder-era and have no established relationship to the dimensional structure the Elders built. They did not exist inside that structure. They existed before it, around it, or in opposition to it — and the distinction matters.
The primary evidence for ancient powers comes from pre-Compact excavations in Realm Seven, where structures were found that appear to be responses to something — reinforced barriers, essence dampeners of unknown purpose, and a series of inscriptions in a script that predates every known mage language. The inscriptions have been partially translated by the Tessarine Order and describe, in clinical terms, a containment effort. What was being contained is not specified by name.
Specimen research has added a second layer of evidence. A subset of specimens designated class X-Remnant carry biological markers that are not traceable to any known evolutionary line within the eight realms. They appear to be native to conditions that the current universe does not and has never had — temperatures, pressures, and essence densities that would require a fundamentally different dimensional structure to sustain. The implication is that these specimens originated from somewhere, or somewhen, outside the known world.