In Warmachine, 'Jack Warcasters are a type of warcaster who specialize in fielding and controlling warjacks in their battlegroup.

Description

These casters have spells that effect entire battlegroups of warjacks in their control areas.^[1]

'Jack Warcaster Traits

• Battlegroup spells, abilities, and feats: Abilities might be passive like Dawnlord Vyros's Bird's Eye ability that grants battlegroup models 360 vision, or they might need to be cast every turn such as Captain E. Dominic Darius's Full Throttle spell. Extending these abilities to the battlegroup increases leverages the ability with the more warjacks included in the warcaster's battlegroup, and often the abilities benefit the warcaster herself!
• Focus Multipliers: Warcasters normally have a limited pool of focus to allocate to their warjacks. A 'Jack Caster might have spells, feats and abilities that effectively multiply this pool by granting additional focus or by replicating the effects that a warjack might gain from having focus (free charges and running). High Allegiant Amon Ad-Raza's feat, Union, creates focus that he can allocate to his warjacks while Epic Butcher allows his warjacks to charge for free. And Magnus the Traitor's Resourceful ability frees up focus to power warjacks provided he casts his upkeep spells on the warjacks themselves.
• Warjack Mobility: 'Jack Casters increase the speed and mobility of their warjacks, or they might nullify or prevent their warjacks from being Knocked Down. Captain E. Dominic Darius can pick up a knocked down warjack with his crane, and Dawnlord Vyros can use the Mobility spell to increase the movement of his warjacks as well as grant them Pathfinder.
• Crowd Control: Warjacks are notoriously easy to tie up in combat with swarms of infantry. Karchev the Terrible can cast Ground Zero to clear himself from a nasty infantry swarm, and Captain Bartolo Montador's feat Typhoon locks down any model willing to move within his control area.
• Focus: A rare feature of your 'Jack Caster is for the warcaster to have a large Focus stat to both allocate focus and have a large control area. Commander Adept Nemo has a high Focus stat that can be extended to 10 Focus due to his Arcane Accumulator.
• Defensive Measures: With fewer models on the field to run interference, 'Jack Warcasters tend to have defenses commensurate with their need to operate more closely to the enemy. Feora, Protector of the Flame gains an armor bonus for being near to her battlegroup while Escort is up. Captain Jeremiah Kraye simply uses his high SPD and mobile force to be close enough to the action but far enough away in safety.

Top Faction 'Jack Warcasters

• Cygnar: Cygnar is spoiled for choice for its 'Jack Warcaster selection, but perhaps the most recognizable one would be the mechanic turned officer Captain Dominic E. Darius. His abilities keep his warjacks operational, forcing the enemy to spend 1.5x to 2x the effort in order to eliminate his warjacks.
• Protectorate of Menoth: High Allegiant Amon Ad-Raza represents Menoth in the field of 'Jack Casting. His spell list and feat favor melee warjacks, and he can complement them with combat skills of his own.
• Khador: Take a warcaster and stick him in a warjack. You get Karchev the Terrible. By Towing heavy warjacks, Karchev delivers SPD 4 ARM 20 heavies where he needs them to be, and further enhances their destructive capacity with focus multipliers with Unearthly Rage.
• Cryx: Helljacks and Bonejacks are already fast. Master Necrotech Mortenebra can make them faster, more accurate, more flexible, and more durable.
• Mercenary: Magnus the Traitor rocks the 'Jacks with his free upkeep spells on the battlegroup, the ability to field his custom Renegades, and his feat that gives them extreme mobility.
• Retribution of Scyrah: Dawnlord Vyros delivers the warjacks into combat with the Mobility spell and the Perfect Execution Feat, or he can use the warjacks to deliver himself and gain the Flanking Bonus in combat.

References

1. ¹ "Newb Q: Are Mercs an actual faction?"