3rd Person ejects Buyer from land bought from Seller; Mandamus or Specific Performance?

BILLY bought a 200-square-meter (200-sqm) parcel of land from TINDAH. TINDAH asked BILLY to choose which portion the latter would occupy out of his vast 1000-square-meter land. BILLY chose the southeast part and, the following day, constructed a house and fenced the area.

A few days later, ISTOR came to BILLY and demanded he vacate the land, alleging better title. BILLY refused, invoking his notarized deed of sale with TINDAH. ISTOR sued BILLY for ejectment.

(a) What is the difference between an action for specific performance and a petition for mandamus?
(b) What is the remedy of BILLY?

(a) An action for specific performance is based on a contractual obligation that the complainant-obligee wishes to have performed by the respondent-obligor. A petition for mandamus lies to compel a government agency, office or officer to perform its ministerial duties under the law.

(b) The remedy of billy is to file an answer to ISTOR's complaint and to file a third party complaint against TINDAH to activate the law on warranty against eviction under the law on sales.