Truncate or delete all records in a custom object

You can't perform any DML using SOQL. The 'Q' stands for 'Query'.

If you want to use Execute Anonymous, then you can use:

delete [select id from myobject__c limit 10000];

However, this has a 10,000 row limit, so that might not be appropriate in all cases.

Salesforce can query 50,000 records at a time but due to governor limit it'll allow only 10,000 DML operation

The only way to perform a quick truncate would be to use the UI to initiate the truncate.