Several methods exist to control undead
Just to be clear, a creature that holds the rod of undead mastery (Magic Item Compendium 175–6) (10,000 gp; 3 lbs.) doubles its available Hit Dice for all methods it uses to control undead. For example…
- The 2nd-level Sor/Wiz spell command undead [necro] (PH 211) doesn't measure the number of undead that can be controlled by the undeads' Hit Dice. The spell is unaffected by the rod.
- The 4th-level Sor/Wiz spell animate dead [necro] (PH 198–9) allows controlling up to 4 HD of undead per caster level that the caster animates with the spell. The rod increases this to 8 HD.
- The 7th-level Sor/Wiz spell control undead [necro] (PH 214) allows the caster to control up to 2 HD of undead per caster level. The rod increases this to 4 HD.
- A neutral or evil cleric (and some other creatures) may rebuke undead (PH 159–60) to such a degree that he commands them. Normally, a creature can control up to HD of undead equal to its level in the class that grants the supernatural ability rebuke undead. The rod doubles this HD.
In addition to these, magic items, other spells, other class features, and even some monster special abilities control undead and measure that control in HD. The rod doubles the HD that can be controlled using all these methods… while the rod's held.
This DM would rule that letting go of the rod leaves some undead uncontrolled
The number of Hit Dice of undead a creature can control doubles only when the creature holds the rod of undead mastery. The rod's description says this twice.
However, as this answer implies, this could mean literally nothing: if the rod increases the number of HD of undead a creature can control using an effect while the rod's held, dropping the rod shouldn't change how those effects were already used! This reading, though, leads to the possibility of handing around the party's one lone rod of undead mastery to each party member individually who can—through whatever means—control HD of undead… essentially doubling the entire party's HD of controlled undead yet dividing the price for being able to do so by the number of party members! ("You get a scroll of animate dead! And you get a scroll of animate dead! And you…!")
Minionmancy is already one of the game's most time-consuming strategies, and this DM would be extremely hesitant to run the rod's effect that way, even if it is technically accurate.
Instead, this DM would have a creature's number of HD of controlled undead return to normal when the creatures abandons the rod… and have that mean that the creature's control over some its HD of undead just ends. This reader imagines three ways a DM can adjudicate this:
- The creature picks which undead remain under its control.
- The undead that have been controlled the longest are freed.
- The undead that were controlled most recently are freed.
(Other options may certainly exist!) Were a player in this DM's campaign considering for his necromancer PC a rod of undead mastery I might poll the table. (My players are a magnanimous, thoughtful bunch.) Were this DM considering for an NPC necromancer antagonist a rod of undead mastery, he'd go with the third option. (That is, this DM tries to make things easy on himself, so he'd want to go with the first option, but the second option is sort of like what happens when the caster of an animate dead spell animates too many dead, and the third option actually makes to this DM the most sense and makes for better plots: the last undead is usually the skeleton army or the unholy colossus or whatever, so breaking the rod frees it from the antagonist's control. Yay, Team Protagonist. Maybe.)