Yes, there are cases when we have a situation where we have to write test cases without having any concrete documents.
In that case, the best way is to:
Collaborate with the BA and development team.
Dig into mails which have some information.
Dig into older test cases/regression suite
If the feature is new, try to read the wiki pages or help of the application to have an idea
Sit with the developer and try to understand the changes being made.
Based on your understanding, identify the test condition and send it to BA or stakeholders to review them.