The clue is the last one that you get, once you complete the main crown tundra game and you can pick up the last legendary clue. The clue, ????????, says “ A m...ious I...ge h..le that con...cts o...r w...ld to ano...r. This phe...m...non was ori...ally dis...ered by Prof...r M...n. The h... co...ts to wor...s whe... Ult... ...sts li...e. Th...e are be...s t...t are diff...nt fr...m the P...ém...n th... we ...re fam...ar w...th.” The description says “A mysterious clue that Peony seems to have dropped. There is a photo attached to it showing a shining hole opening above the Max lair.” You get it after the reins of unity.