The lich can be summoned by the temple. It doesn't cost alot of mana and like other undead units can be made quite quickly. It is a melee tier 3 unit and it requires the use of its personal mana in order to attack. It's attack produces an area of effect (aoe) around it, dealing damage to everything, including it's allies, and even to other liches, but this attack deals less damage to other undead units, like liches, zombies, skeleton archers, rictuses and the Dark Hand. They are very slow, but can swim in the waters, allowing to invade islands. It combines well with rictus units as they can traverse water too.
The most effective way to use the lich is to combine it with other undead melee unit (zombies or dark hand, so they will cause less damage to the player's own troops. this is specially true in case of the Dark Hand units, as they can turn corpses into more liches to support their assault. Liches also provide good defense to skeleton archers. Use cover such as rocks in order to force the ranged enemies to come closer and, once they are in range of the aoe, force the lich to attack in front of itself so that it gets a few hits in, and possibly a few kills as well, before the enemies are in attack range/sight.
Overall, the lich is a support creature, not to be used in numbers but to provide extra damage dealt by other undead units. However, when fighting other undead or construrcts it is kind of a useless creature, but it can be good at dealing with lots of lesser units or shooters.