As mentioned in Super-Fight Week, v2.9 patch introduced General Secret Temple for low-level characters. This is basically the same map as the old Secret Temple (aka High-Grade Secret Temple), and is accessible via the warp gate in Thueringen Lakeside at 12 noon (channel 1 ONLY), 8 pm, and 10 pm (server time).
This low-level Secret Temple limits characters up to Lv.90. (The article linked above says Lv.80, but that is wrong.)
The boss is Hill Giant (Lv.80), and the mini-boss is Wahlansche (Lv.78). Like all Super-Fights, the boss spawns 30 minutes after the opening of the warp gate in the middle of the map.
The boss drops 2 Old Hero's Seals, while the mini-boss drops just 1 Old Hero's Seal. You can trade 3 Old Hero's Seals with the NPC at the Super-Fight warp gate for 1 Hero's Seal, which is needed for Enhanced RES Necklace Recipes and Selva's Recruitment Quest.
Other loots include Lv.84 enchantment chips, Lv.80-88 equipment, and Lv.92 unique weapons. The regular mobs drop Sun Stones (1,000 feso each).
... & Sword of the New World
This v2.9 feature is not yet available in Sword of the New World, since K2 still refused to reveal ETA date for the rest of v2.9.