"Null" as a call no longer exists. It has been merged with Pure Shard.
If you have Pure Shard about your person you cannot use essence and you are immune to all essence used on you. If you are a magical creature (Fey, Elemental, Elf, Demon, most unique races) being in contact with Pure Shard hurts. Discarding the Pure Shard makes you instantly vulnerable to negative effects but you remain unable to use essence and immune to positive effects for 5 minutes.
If you are struck by Pure Shard then all active spells are dispelled.
If you take damage (after armour/dex is taken into account) then you cannot use essence for 30 seconds (this is not cumulative, it's 30 seconds after the last time you were hit).
Dodge takes place before you are hit so you can use essence to dodge the blow thus avoiding both of the above.
Pure Shard does double damage to magical creatures.
Star Tear does double damage to werewolves.
Gravenail does double damage to undead.
"Rift" as a call means that the spell/blow/effect ignores Immunities and resistances.
"Overchannel" means the spell/blow/effect ignores immunities only. Resistances can be applied as normal.