Just to let you know after the second form, Lich-Born, is destroyed the original form does not show to re-deploy/cool-down. This was tested in a PvP casual and PvBot Would put post in Bug forums but......
Strange, because the opposite happens with Korsein: when the second form drops, the first form is shown to be cooling down and can actually be redeployed while the second form is still in play. I assumed Dark Knight worked the same. Must be a bug with Lich-Born specifically and not Rider abilities.