I'm not convinced that "an ancient evil awakens" is referring to the Didact. If you read Cryptum and Primordium, anybody would probably agree that he isn't "evil" in the black-and-white terms the sentence implies. I mean, he's ancient in relative terms, but in comparison to something like the Timeless One? Not so much (though I'm not saying it is the last Precursor). Someone said that his problems with humanity are more political than sentiment, and I'd say that's an accurate assessment.
In comparison to what 343 could be doing showing gameplay trailers of him in action, dedicating entire trailers to expanding his story and introducing him to the audience we're getting very little; only his symbol. In fact, the only time we've seen the Didact in any form other than his symbol was the trailer released recently in which he is holding the Chief hostage, but that doesn't show the full context. We don't know why he's holding the Chief hostage.
Of course, I could just be seeing what I want to see.
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The Didact symbol is a lovely avatar theme for Halo 4. Looks great!