The card states: "Test agility 4. If you fail, take 1 damage and 1 horror. Then disengage from each non-Nightgaut enemy engaged with you and move to a central location".
Question: if I succeed the test, do I disengage and move or do I stay where I'm `?
The use of the word "then" requires that the preceding conditions have been filled. If you succeed, you don't take the damage and horror, so you don't move onto the "then" instructions either.
Would be convenient sometimes, though.
I sometimes wonder if this treachery takes the prize for "Most Frequently Failed On Purpose."
I always read this as:
Test agility 4
If you fail, take 1 damage and horror.
Then disengage...etc.
The preceding condition being the agility test itself, not the fact that you failed it.
"Then" as a mechanical term only looks at the text immediately preceding it, which in this case is the damage/horror. In your example layout, it would be indented under the damage/horror line.
Here's the Arkhamdb.com entry: