Mistform Ultimus

• WWizards of the Coast2006-09-25
  Official Ruling
  The type “Illusion” is printed on the card’s type line purely for flavor. The Ultimus has every other creature type as well.
• WWizards of the Coast2006-09-25
  Official Ruling
  Like any creature, if Mistform Ultimus becomes another type as the result of a spell or ability, it will lose all its other creature types.
• WWizards of the Coast2006-09-25
  Official Ruling
  Mistform Ultimus has all creature types even if an effect removes its ability. Type-changing static abilities will apply in layer 4 before effects such as Sudden Spoiling’s remove them in layer 6.

Rulings via Scryfall