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Modtweaker’s Thaumcraft integration extends IEntityDefinition.
That means that you can use the methods below on any IEntityDefinition object:

Set aspects

You can use this to override what aspects an entity would normally have.
Requires a CTAspectStack[] or many CTAspectStack objects.

<entity:sheep>.setAspects(<aspect:terra>*5);

Remove aspects

You can use this to prevent certain aspects from being added to an entity.
Requires a CTAspectStack[] or many CTAspectStack objects.

<entity:blaze>.removeAspects(<aspect:ignis>);