The purpose of this paper is to demonstrate a more retentive custom modified lock and key design of metal cast post and core for the restoration of grossly destroyed endodontically treated molar tooth. The lock and key metal cast post consists of two parts, one in the distal canal (primary post) and the other one in mesio-lingual canal (secondary post). The primary post has a lock design, while the secondary post contains the key design, both of which interlock together. Lock and key cast post, mentioned in this report can be an effective design for the management of grossly destroyed molar teeth.