Leadership is a special (or natural) ability of Warlords.Erf-b1-p040a It grants all units under direct command of a warlord a bonus to their stats.Erf-b1-p084a
Leadership bonus of Chief Warlord applies to all units of their capital.Erf-b1-p040a Having the Chief Warlord in the same hex adds another bonus. Units in the stack led by Chief Warlord get yet another bonus.Erf-b1-p113
Known Leaderships & Comparison
2 - Parson Erf-b1-p040a This apparently is a very unimpressive number, especially for a Chief Warlord.
2 - Dora Erf-b1-p050 (This may be a reference to level rather than leadership bonus)
5 - Sir Webinar Erf-b1-p050 (This may be a reference to level rather than leadership bonus)
9 - Jillian Zamussels Erf-b1-p050 (This may be a reference to level rather than leadership bonus)
10 - Prince Ansom Erf-b1-p145 This is a reference to level, not leadership bonus.
The bonus is directly proportional to the warlord's level.
Apparently the leadership bonus is very significant (and thus very important), since leaderless stacks are relatively weak.Erf-b1-p058, Erf-b1-p113, Erf-b1-p125, Erf-b1-p128
"Leadership" can also refer to a either a single leader or group of leaders such as Warlords or Casters. Gobwin Knob sought to target or "hit" the leadership of the Royal Crown Coalition to deny RCC forces leadership bonuses. They were successful when they hit RCC Leaders Duke Nozzle and the Blue Warlord.