Those yellow crystals are called revival crystals which are used to buy the revived unit at the trading post.

The revival event itself usually lasts for one week and the units stay for an extra one week before or after the revival. Well, the revived unit will be in the trading post whenever their revival event is up, so I guess you could call it a rotation.

As mentioned above, those units are old event units. Meaning you can get units like Anya, Karma, Metus, and other star trial and item collection units. In other words, some are good and some are not worth it. Anya is definitely worth it because black princess and 2-block.