Alter and Elven Fruits are resources in Legacy of Discord with which you can improve the talents of your Metamorph Masteries.
They got released together with the Metamorph update in 2018 and appear to give great benefit to the skills of your character when transforming to a specific Metamorph.
You can access the Metamorph Mastery menu and browse the different Metamorph talents by tapping on the Metamorph button and then on Spirit Codex -> Mastery button of a specific Metamorph chosen, as below:
After tapping on the + button on the top right, you may increase the Mastery points of a Metamorph and use it to increase talents of your choice.
Alter fruits are used for higher-level Metamorphs while Elven fruits are used for lower-level ones.
A good tip is to increase talents that are used to offer you increased effects of Stun, Paralysis and Immunity.