A contract in which one party promises to indemnify another against loss that arises from an unknown event is:

A restoration policy typically refers to a type of agreement focused on restoring property to its previous condition rather than providing indemnification for losses due to unforeseen events. While it may cover restoration costs, it does not inherently promise to indemnify against losses from unknown risks, making it less applicable in this context.

A retrocession agreement involves the transfer of risk from one reinsurer to another, essentially a secondary form of insurance in the reinsurance industry. This type of agreement does not provide indemnity to an original insured party against losses from unknown events, making it irrelevant to the question's context.

A hold-harmless agreement is a contract where one party agrees not to hold another party liable for any damages or losses incurred. While it can offer some protection, it does not constitute an indemnity against losses from unknown events as an insurance policy does, which specifically covers unforeseen occurrences.