Incorporeal Movement. The spirit can move through other creatures and objects as if they were difficult terrain.

Possession: 

The familiar possesses one undead visible within 5 feet of it; the familiar disappears, and the adjacent undead loses control of its body. The familiar assumes control of the body, sustaining it with necrotic energy. The familiar can't be targeted separately by any attack, spell, or other effect, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed undead's statistics, but doesn't gain access to the target's knowledge, or class features. Your minion uses your proficiency bonus rather than its own. In addition to the areas where it normally uses its proficiency bonus, an undead minion also adds its proficiency bonus to its damage rolls. The undead minion is immune to effects that would turn or destroy undead, thanks to the strength of their connection to you.

The possession lasts until the possessed undead drops to 0 hit points, the familiar ends it as a bonus action, or the familiar is forced out by an effect like the dispel evil and good spell. When the possession ends, the familiar disappears, leaving behind no physical form. It reappears after you cast the find familiar spell again.