Flash-Cloning

Summary
A piece of UNSC technology, a flash clone is a quick, incomplete clone. An embryo is taken and developed a hundred times faster than it would have naturally, becoming something different than it would have been. Many anomalies appear because flash clones are forced to go through time too quickly. Flash clones don't have any memory, no training like natural humans get observing people while growing up. They can be trained with intensive therapy, but after a year or two they start to degenerate from metabolic instability until they die through a process called Metabolic Cascade Failure.

After the future SPARTAN-IIs were kidnapped from their homes, they were replaced by flash clones.