If you don't understand, Lucretia specializes in vampires. At high levels, her vampires are very strong.
And I don't know why you brought up 100 Titans here. The game lasts no more than 2-3 months, maybe 4 on very large maps.
Are you going to gather 100 Titans, smart guy?
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There are some playstyles where the Swarm is useful; in some of them, it's very effective.
For example, you build an army of tough, defensive creatures that can hold the line, and the Swarm casts spells non-stop (like an "outlast" strategy).
And it also works against Inferno, which relies on Imps to drain all the mana; here, they get a rude awakening.