1. Core Legal Answer & Context: "Substituted performance of contract" is a significant remedy introduced into the Specific Relief Act, 1963, by the Specific Relief (Amendment) Act, 2018, specifically in Section 20. This provision offers an alternative to specific performance, allowing an aggrieved party, in case of a breach of contract, to get the contract performed by a third party or by their own agency, and then recover the expenses and costs incurred from the defaulting party.

Key aspects of Substituted Performance:

• Alternative Remedy: It is an alternative to seeking specific performance from the court. Once the aggrieved party opts for substituted performance, they cannot claim specific performance of the contract.
• Notice Requirement: Before resorting to substituted performance, the aggrieved party must give a written notice of not less than 30 days to the party in breach, calling upon them to perform the contract within the specified period.
• Failure to Perform: If the party in breach fails to perform the contract within the period specified in the notice, the aggrieved party is then entitled to get the contract performed by a third party or by their own agency.
• Recovery of Costs: The aggrieved party can recover the costs and expenses actually incurred by them for such substituted performance from the party in breach.
• Without Prejudice to Compensation: This remedy is available without prejudice to the right of the aggrieved party to seek compensation from the party in breach for any other loss or damage caused by the breach.

The legislative intent behind this amendment was to reduce the burden on courts for specific performance decrees and provide a more practical and efficient remedy for the aggrieved party, especially in commercial contracts.

2. Relevant Statutes and Sections: Section 20 of the Specific Relief Act, 1963, as amended by the Specific Relief (Amendment) Act, 2018.

3. Important Landmark Cases: As this provision was introduced by the 2018 amendment, specific landmark cases interpreting "substituted performance" are still evolving. However, the amendment reflects a shift towards a more damages-centric approach in contract law, aligning with international commercial practices.

4. Clear Conclusion: Substituted performance, introduced by the 2018 amendment to the Specific Relief Act, empowers an aggrieved party to get a breached contract performed by a third party or themselves after due notice to the defaulting party, and subsequently recover the costs. This provides a practical and efficient alternative to specific performance, aiming to streamline dispute resolution and ensure timely contractual fulfillment.