While it may be difficult to find documents containing high modality words specifically intended for essay use, it may be slightly easier to compile a list of high modality words (the aim of which is to indicate certainty, forcefulness, intensity and strength) to be used in essays, as follows:
- must
- ought to
- shall
- has to
- have to
- absolutely
- always
- certainly
- never
- definitely
- undoubtedly
- unquestionably
- will
- will not
- clearly
- evidently
- surely
- entirely
- completely
- totally
I hope this helps!