Skill Combination Effects

Some heroes' skills can be combined to create an additional effect. (These were all I could remember, I would appreciate assistance with the rest. -siege)

United Creeps

Nightmare + Spire

• Units hit by spire under the effects of nightmare will be frozen for a short amount of time and damaged (broken in 1.12)

Encouragement + Rebirth

• When a unit enchanted by rebirth is sacrificed by encouragement a flying unit is spawned from it's corpse

Nightmare + Howl

• Waking units from a nightmare with howl deals damage and instantly wakes them

Elven Brigade

Flame of the Devout + Dash

• Dashing through the flame leaves a burning trail behind that deals damage to enemies that come into contact with it

Backfire + Syphon Sigil

• Using Syphon on a hero with Backfire on it will cause the Glyph to drain mana and deal damage instantly rather than over a short period

Maelstrom + Incinerate

• Incinerating the tornadoes created by Maelstrom will light them aflame, adding unit damage to the existing building damage effect

Grim Brigade

None of them are known currently.