The Elemental Cradle is a place where you can battle hydras in your Guild. You need your team to be Team Level 35 to go in there. This event can only be seen and done on MOBILE.
In the new mode, you have to challenge a new boss – the multi-headed Hydra. Hydra has several difficulty modes. As the difficulty increases, so will the rewards for victory. Each of Hydra’s six heads has unique characteristics, which means you need to find a different approach to each one and plan your attack strategy together with your guildmates. The more heads of Hydra you defeat – the more rewards each guild member receives. There are 5 hydras which are: The Darkness, Light, Earth, Water, Wind, and Fire hydras. This battle requires teamwork, so you need your guild members.
Source of Information: https://www.herowarsmobile.com/hydra-battle-tips/
There are different heads to choose on the hydra. These are the elder, ancient, dreadful, common, and legendary hydras.
To fight the hydras, you need special fangs as well.