There has been recurring questions about the Underhand of the Emperor's first ability, which seem to have occurred partly because I may have misspoke occasionally, and partly because people have made some unwarranted extrapolations. I hope this will eliminate any remaining confusion.
If the first ability of Underhand of the Emperor is used on a Dishonored Personality, the effect fails, so his controller must attack and you assign the Personality.
If the first ability successfully causes an attack, the privelege of assigning the target unit completely and totally transfers to you. The unit's controller does not get to assign that unit during the normal attack phase this turn at all.
Because you have the right to assign the unit, you can opt not to assign it anywhere, which results in leaving the unit at home (just like any other unit you control). This is not "assigning the unit to home." Units are only assigned to battlefields. (Additional: This ruling has reversed; see here.)
After assignment, the unit's controller may move it between battlefields, home, etc., as normal.
Additional: Previously, I said only a normal attack would fulfill the requirement the Underhand of the Emperor creates. I have since found a relevant counter precedent: Rikugunshokan. Note that ruling is a few years old, so may have a higher probability than normal of reversing. We'll see if it causes more than the usual number of problems. For now, attacks created through card or stronghold effects can be used to meet the requirement imposed by Underhand of the Emperor.