The "Marshall Undead" ability is much vaguer. All we know is its range.
My first thought was to model it on a Harpy's Song:
Marshal Undead (Su) [Captivating]: Once a day, a bone tyrant master can call undead to gather towards it. All undead (other than bone tyrants) with 20 miles of the master must succeed at a DC X Will save or be marshaled. An undead that successfully saves cannot be affected again by the same master's marshal undead call for Y. The save DC is Charisma-based.
A marshaled undead walks toward the bone tyrant master, taking the most direct route available. If the path leads into a dangerous area (through flame, off a cliff, or the like), that undead gets a second saving throw. Marshaled undead can take no actions other than to defend themselves, but this "defense" can include attacking creatures that attempt to injure or obstruct the victim. The effect continues for as long as the tyrant concentrates and for 1 round thereafter. A cleric's turn or rebuke undead ability allows the marshaled undead to attempt a new Will save.
If I recall accurately, Freyar fancies a "teleporting" version, something like:
Marshal Undead (Su) [Teleport]: Once a day, a bone tyrant master can call undead to gather towards it. All undead (other than bone tyrants) with 20 miles of the master must succeed at a DC X Will save or be teleported (as per greater teleport) to an clear area within Z feet of the bone tyrant master. An undead that successfully saves cannot be affected again by the same master's marshal undead call for Y. The save DC is Charisma-based.
However, upon reflection I'd prefer to use a version that relates to the tyrant's Subjugate Undead ability, and I do like the "wandering across the land" element of the first version.
Marshal Undead (Su) [Subjugation]: Once a day, a bone tyrant master can call undead to gather towards it. This is resolved like a use of the tyrant's Subjugate Undead ability (see below) that can affect any undead within 20 miles of the master except for other bone tyrants. Marshal Undead does not require line of effect or line of sight. Affected undead start walking towards the bone tyrant master, taking the most direct route available. A marshaled undead will try to move around or break through obstacles, and will attack creatures to threaten or obstruct them. If the Subjugate Undead check had a "rebuke" result, the undead will walk towards the master for 1 hour and then revert to its normal behavior; undead Subjugated by a "command" result will keep walking until they are destroyed or a bone tyrant instructs them to stop. A cleric may channeling channeling positive energy or negative energy to dispel a Marshal Undead effect. This follows the "Dispelling Turning" rules for an evil cleric except there are no alignment restrictions.
...come to think of it, I'd also add
A bone tyrant master can marshal up to X Hit Dice of undead at any one time. These undead count towards its Subjugate Undead Hit Dice limits. A bone tyrant (master or minion) can determine the location of its marshaled undead (as per the locate creature) spell) as an at-will ability.
That way, the master can send out its minions to use their
dimension door ability to extricate undead who have been marshalled but are unable to move (sealed in a tomb, blocked by a river, cliff or other barrier, et cetera).