If you can Repeat the Power "this turn" (e.g., from "Powestorm" or "Gift of Strength"), then you may use another Power between the two uses of the repeated Power.
If the Power repeats itself (e.g., "Creepers Tear into Mortar" or "The Land Thrashes in Furious Pain") or you are granted the option to Repeat a Power "After" using it the first time (e.g., from "Relentless Punishment" or "Strength of the Waking Island"), then resolve the Power and anything else it triggers (e.g., "Vengeance of the Dead") before immediately Repeating it – you don't have a chance to use another Power first. (Discussion elsewhere) (Discussion elsewhere) Tags: Repeat, Gift of Strength (Vital Strength of the Earth), Powerstorm, Gift of Twinned Days, 20 more...
Use the Power as though the number in the "Range" box were increased or completely ignored (depending on the Range boost in question) and the extra target requirement were included in the "Target Land" box. In the case of Shadows of the Dahan, this means ignoring the Range number entirely (but not anything else in the "Range" box!) and adding "Dahan" to whatever else is already in the "Target Land" box.
The initial use of the Power targets a land and resolves (along with any non-Repeat actions it triggers).
This triggers both "Relentless Punishment" on that land and the Power's own Repeat instruction. Since the order is ambiguous, you choose which one happens first.
After the Power Repeats itself, "Relentless Punishment" triggers again on the second land targeted.
Because "Relentless Punishment" counts allprior uses of the Power, using that special rule a second time can get quite expensive (Power's Energy Cost + 1 Energy for the first use + 1 Energy per initial "Relentless Punishment" use + 1 Energy for the self-Repeat use)!