Undead dragons are rarer than most dragons this is what happened when a dragon dies and abundant of raw magic energy is in the area and resurrect the dragon in an undead form. they keep some of the traits from there old form. 

The most cunning and treacherous of true dragons, green dragons use misdirection and trickery to get the upper hand against their enemies. A green dragon is recognized by the crest that begins near its eyes and continues down its spine, reaching full height just behind the skull.

On initiative count 20 (losing initiative ties), the dragon takes a lair action to cause one of the following effects; the dragon can’t use the same effect two rounds in a row:

• Grasping skeleton hands erupt in a 20-foot radius centered on a point on the ground that the dragon can see within 120 feet of it. That area becomes difficult terrain, and each creature there must succeed on a DC 12 Strength saving throw or be restrained by the hands. A creature can be freed if it or another creature takes an action to make a DC 12 Strength check and succeeds. The skeleton hands wilt away when the dragon uses this lair action again or when the dragon dies.
• The dragon make shake the ground causing stalagmites to fall from the sky. these stalagmites contains 1d6 skeletons in side them that get added to the initiative order at the liar action.