Energy Siphon is a Defensive Ability in Titanfall 2, available for the Monarch chassis. It functions similarly to the Laser Shot used by Ion though, instead of dealing direct damage, it "steals" a victim's shield. It will also slightly blind the enemy and slow reaction time, making this incredibly annoying to enemy titans. If an opponent already has shields, it will take away a large amount of their shields (but will still give you the default amount).
Energy Siphon can be upgraded with the Shield AmplifierKit, to make the shield gained 25% stronger.
It can also be upgraded via the Upgrade Core. Energy Transfer, available at Tier 1, will allow Monarch to play Team Medic by providing friendly Titans with energy shield when shot by the Siphon. Energy Field, available at Tier 2, will turn the Siphon into an Area-of-Effect (AOE) weapon, increasing effectiveness against large clusters of enemies.