Psychic Spark adds an extra dart to each Magic Missile cast (e.g. four total at spell level 1).
When interacting with effects like Death Ward and dealing multiple missile attacks to the same creature, assuming the 1st attack was sufficient to down the creature with Death Ward active, the 2nd attack appears to apply before Death Ward activates. This means that after 2 missile attacks the creature loses Death Ward but remains with 1HP:
If the initial attack has 3 missiles the creature is downed on the 2nd attack and again for an actual kill on the 3rd attack.
In the case of party members with Death Ward applied, they need to receive 6 missile attacks to actually die: 2 to break down the ward, 1 to get downed and 3 more to fail 3 Death Saving Throws. This effect appears to apply to all multi-application attacks such Scorching Ray and Eldritch Blast (as of 4.1.1.3669438).
The incantation for Magic Missile is Tormentum, Latin for "Catapult", "Missile", and "Torment".