As the name makes it obvious, the spell causes a tornado to descend upon an area, sucking all creatures inside it and throwing them around in the air. Even flying creatures are generally unable to break free before the tornado disperses. Once it does, its 'contents' are thrown back onto the ground, dealing heavy fall damage in the aftermath.
Tornado is an Ultimate spell that creates a large tornado that moves randomly on the map, and it can suck up units and wizards and stun them for its entire duration, until it ends, dropping them on the ground, causing potential fall damage.
Tornado has a very long and noticeable cast and can be interrupted, and easily avoided if noticed.
Enemy wizards and their army are good and easy targets for tornado while the wizard is building a manalith.
You can also try casting Tornado close to the edge of a map. If you succeed, there is a good chance that some of the creatures sucked in by the tornado will get thrown into the void afterwards, thus the enemy will lose them permanently.
Wizards and units will be rendered useless for a long time with Tornado - giving you plenty of time to destroy important structures and units.
Because Wizards will be helplessly stunned in the air while in a Tornado, they cannot heal and protect themselves from the effects of desecration, making a desecration very easy with a well placed Tornado.
There is a related bug, however, with the spell: although the tornado will very rarely actually go into the void, occasionally it can if there is, for example, a very narrow walkway. If this happens, all creatures on the map will get thrown up into the air and tossed around, as if suffering from the effects of the tornado. Be warned!