When the High Templar attacks an enemy, a buff is added at that enemy when the missile hits.
After a few seconds, that buff searches for the same High Templar and launches a missile back at it.
Similar principle as how the Crypt Fiends in Warcraft 3 function - a missile is launched from the Crypt Fiend at the enemy and then the missile returns.
I thought this would be easy - the High Templar adds a buff at the target upon impact. That buff has a Search effect that searches the entire map. That Search effect is linked to a Set effect that uses a validator that checks for the caster. Then the Set effect launches a missile, targeting the unit specified by the validator... The problem is that no validator seems to be able to check for the caster... I can make the missile return to ANY friendly High Templar on the map, but not return to the actual caster.. : /