The barrier is primarily formed of mage-energy, of course, but that isn't the main form it decays into. Pure mage-energy is less useful, in this case. The immortal one shields against it, for one, and also it's not actually that good at penetrating thick stone.
Vkandis can also work with light. Perfectly ordinary, non-magical light, that Leareth has no reason to specifically shield against.
There is inevitably some overflow in low-energy, long-wavelength light, also known as "heat", along the current surface of the barrier. It's a minuscule fraction of the total energies in the barrier, for obvious reasons – it's not very damaging miles underground or miles in the air, where it either pointlessly half-melts some rock or dissipates quickly in the air - but it does, of course, set off a mile-wide ring of burning forest around the entire perimeter of Iftel's border.
The vast majority of the energy exits mostly as extremely short-wavelength light, aimed west and originating from the upper component of the barrier's surface, at a shallow angle. It leaves a bar of brightly-glowing ionized air in its path, and then a substantial component of intersects with the surface of the conveniently very flat tundra a hundred or so miles away. (This is relevant because there isn't, actually, enough energy in the barrier to completely vaporize all of the stone in a straight line for over a hundred miles in a fraction of a second.) A smaller but still substantial fraction exits as slower-moving mage-energy, already in the process of decaying into more high-energy light.
A lot of it "misses", of course, the beam having spread out a little while it traveled; the brunt of it lands on about a square mile of surface rock, and only some of that contains Leareth's base. The stone attenuates the remainder – a lot more than the magical shielding does, actually. Even gamma rays can't trivially penetrate ten feet of solid rock - though the beam, even at this distance, is intense enough to vaporize the first few feet and partially melt the next few. On the surface, things are about to get very exciting.
The thorough earthquake shielding isn't damaged by high energy radiation, though, which passes more or less straight through it. The shielding is designed to handle the stone around it suddenly becoming unstable, and it holds, and the ceiling doesn't collapse, and the strength of the radiation blast makes it through several orders of magnitude weaker. It no longer makes the air glow.
More than half of the energy hits in a fraction of a second, but the rest, that exited more slowly as mage-energy and then decayed, makes it through over the next two or three seconds. By five seconds in, it's pretty much over. The secondary radioactive isotopes from photonuclear reactions are trivial in comparison.
It should, Vkandis thinks, still be a couple of orders of magnitude higher than the minimum high-energy light it takes to guarantee the squishy mortals will die.
(There aren't that many interesting non-magical-radiation phenomena in Velgarth, not in places where mortals frequent, but Vkandis finds light very interesting, like the Star-Eyed finds Heartstone stability magic interesting, and has paid attention.)