Delete databases from oracle 12c I want to delete multiple databases and its entries. These instances are not getting started. I want to delete data files also.

EDIT: I am thinking delete all data files. Please help on below concern.

Shall I delete data files manually? Will it cause any problem to my working database instance?
The easiest and safest way to delete a database is using the RMAN DROP DATABASE command.

If on Linux, set up your ORACLE_HOME and ORACLE_SID environment variables appropriately then start the database(s) using startup mount exclusive restrict.  Then drop it using RMAN's DROP DATABASE.  If you want to delete the backups/contents of the FRA as well, then use DROP DATABASE INCLUDING BACKUPS.

You could also use the GUI (DBCA - Database Creation Assistant), but this may be trickier depending on the environment.