In the latest expansion content, the system for "Latent" and "Renewed" Sunder Charms has become more gated and complex, focusing on a new mechanic called Heralds of Terror:
The Herald System: Instead of just farming any Elite in a TZ, players now hunt Heralds.
Drawing "Ire": As you clear a Terror Zone, you may see a message that you have "drawn the ire of the Herald."
Herald Tiers: Heralds spawn in tiers (1 through 5). Each time you kill a Herald in a single game session, the next one that spawns will be a higher tier.
Best Drop Rates: While Heralds of any tier can technically drop a Sunder Charm, the drop rates are significantly higher (and some types are locked) to Tier 4 and Tier 5 Heralds.
Session Persistence: To get to Tier 5, you must stay in the same game session and continue clearing Terrorized areas. If you leave the game, the tier resets to 1.