an orb effect is basically an ability that changes the attack art of the unit. It also usually adds extra effects (ie orb effects) like lifesteal, cold damage, etc.
to make one, just base an ability on something that is an orb? some orbs are
Orb of Slow (chance to cast an ability on attacking)
Orb of Lightning (Same as orb of slow)
Orb of Frost (adds cold damage to attack)
Frost Arrows (same as orb of frost)
Searing Arrows (adds damage)
Orb of Fire (Adds damage, damage is splashed to nearby enemies)
Orb of Corruption (minus enemy armour per hit)
Lifesteal (attack drains life)