Mystery symbols land as blank or question-mark placeholders and, once the reels stop, all transform into the same randomly chosen symbol. If several mystery symbols are in view, they all become the same thing, which can create a large win at once.
The reveal can be any regular symbol and sometimes a wild. Because every mystery position becomes one symbol, games with many mystery symbols can swing between nothing and a full screen of premiums, so the mechanic tends to raise volatility.
The outcome is decided by the RNG when the spin is settled; the reveal animation adds suspense but changes nothing. Mystery symbols are close relatives of symbol upgrades and the xWays reveal. See our mystery symbols feature page.