Repeaters are a once-lost vedalken technology rediscovered during The Demon War. Though their full potential is well understood and voraciously pursued, modern artificers have so far only been able to construct primitive facsimiles.
Repeaters function not unlike spell-storing rings but have many important distinctions.
Instead of storing their spell for later casting, repeaters simply "repeat" the spell as though they had cast it themselves. This process is initiated within an "origin stone" into which the spell to be repeated is first cast. This prime obelisk communicates the spell to subsequent ones that share its identifying sigils.
Crucially, additional repeaters do not need to be targeted by the spell's repeated casting in order to repeat it themselves (as long as they are within the active range of a corresponding repeater). The end result of this network-like behavior is that a single spell can be cast once by an individual and then seemingly infinitely repeated. Given enough repeaters in strategic locations, a spell can conceivably travel an incredible distance in a short time, being cast over and over all along the way.
Unfortunately, although the vedalken were capable of constructing massive repeaters that could handle even very powerful magic, modern imitations have all paled in comparison. The most advanced versions available today are about five feet tall and can repeat only low-level spells.
A Timeless Prison
Repeaters were of the vedalken strategy for imprisoning The Void Song. With an almost incalculable amount of planning, the vedalken were able to contrive an encounter with the creature wherein it was not inspired to split itself apart. Then allowing it to possess a single member of their species (and before that unfortunate soul's demise), the vedalken cast a Time Stop spell into a massive origin stone.
Repeating that spell to a second repeater, which repeated it to a third, which repeated it back into the origin stone, the vedalken created a small pocket where time ceased to flow. The Void Song was unable to consume or depart its host and remained locked in place for centuries. It remained there until a group of adventurers released it to help combat Ozcymath and his horde during The Demon War.
Pranking the Necini
Inibis Valmeia recently oversaw the construction of a repeater network throughout the Bluffs district of Necine. She uses it primarily to repeat the Suggestion spell, wreaking minor havoc in the district through simple pranks like preventing people from making left turns.