Ghostborn is a Spirit affinity champion from the Dark Elves faction. He has a fairly unique ability dropping the entire enemies teams defence with his A3 without needing accuracy. This requires you to make some decisions as the rest of his abilities do need accuracy to work such as his AOE heal reduction and A1 drop defence ability. The answer generally is the scenario you run him, Arena I would usually make him hit hard and remove accuracy as you expect a quick fight and he is in the team to set up a nuker. In Spider it is a similar situation, once drop defence is active he has done his job as you cannot cast heal reduction on the main spider. Other dungeons and Faction Wars you need to decide if the single target decrease defence and heal reduction are useful to beat the content.
Whilst applying the decrease defence ability Ghostborn also increases the attack of your team making him an end game viable champion for speed builds and nuke strategies.
Overall a strong legendary in the right situation.