Theron still believes he's forging something holy out of captured starlight. Cathedral Level 2 has taught him patience, if nothing else - and patience, applied with a hammer, is its own kind of threat.
A visibly telegraphed 3-round Forge starts - he does not press the attack while it runs.
A dedicated OnDamaged hook (cl2_tv_damaged.nss) tallies every point of real damage he takes during the channel.
An AoE divine burst lands on his marked target, and his epic Mage Armor - the Unbroken Ward - answers on himself in the same instant.
The Starshard shatters mid-forge - real self-damage backlash onto Theron himself, and a longer cooldown before he'll attempt it again. No prior interrupt mechanic here punishes the BOSS for a channel the party successfully pressured.
Cathedral Level 2's real ceiling stood at CR19 after this session's own cathedral revamp (three new PRC bosses added earlier this session); Theron was originally built and wired at CR17 into the area's enc07 slot before that revamp landed, and is kept at his original CR rather than retroactively re-pitched.
He is the bestiary's first Knight of the Weave - a Mystra-touched spellsword PRC, cloned from fallenpriest, the same corrupted-priest donor round 6/8 used for their own bosses in this cathedral.
Real Epic Spell: Mage Armor (id 639/877) - reflavored the Unbroken Ward, the defensive payoff of a successfully completed Forge rather than a scheduled autocast.