Naglfar

Naglfar can not be crewed by creatures on the
battle field.
When a creature you control dies, it crews
Naglfar. (It still enters your graveyard.)
When Naglfar attacks return all creatures
crewing it to your battle field.
Crew 20 (When the total power of creatures crewing
this artifact is equal to 20 or more: this Vehicle becomes
an artifact creature.)