From my guide:
Event eidolons (Raids)
Raid event eidolons were made available in the Eidolon Orb shop in August 2017 (DMM version). These orbs can be obtained from selling eidolons: higher rarity and Gatcha eidolons fetch more orbs, and the reward scales 1:1 with number of limit breaks. Kaiser Dragons are also available in the eidolon orb shop (only 1 copy available per element), and they’ve been buffed so that their stats are comparable to LV100 event eidolons at LV40, and their active skill provides a +30% elemental attack boost and +30% elemental resistance (against their strong element). This makes them optimal Burst buffers, and they also provide the strongest defensive summon effect in the game.