[Aaron]

Yes, you can certainly make an eval script which overwrites that. I'd create a class ability to bootstrap to the archetype with description text describing the specifics of how you get multiple kinds of revelations.

Then add two eval scripts to the class ability.

Script 1 will copy the contents of the cCstS3Expr field immediately before the mysteries append their own specific text for their revelations. It can store this in a text field (abText maybe? Possibly that field has a limit on number of characters it can store, not recall off the top of my head).

Script 2 will run immediately after the mysteries append their own specific text for their revelations. Have this one iterate through all mysteries added to the class in whatever table you're adding them, and pull the Custom.Myst? tags to itself, then overwrite cCstS3Expr using the field and the combined tags.

Rough examples
Final 999998

call foctoclass

doneif (state.isfocus = 0)

field[abText].text = focus.field[cCstS3Expr].text

Final 1000000

foreach pick in hero from Ability where "abCategory.Whatever & CustTaken.Whatever"
  perform eachpick.pulltags[Custom.Myst?]
  nexteach

call foctoclass

doneif (state.isfocus = 0)

focus.field[cCstS3Expr].text = field[abText].text & " & (" & tagids[Custom.Myst?,"|"] & ")"

As always, rough code, you'll need to tweak it as necessary (Particularly the "whatevers" in the tag expression for the foreach).