This whistle (made from an oversized acorn), if attuned to you, enables you to control (command) Undead and all parasites/creatures parasitically linked to you.  The CR of the undead/creatures that you can command is limited by your total character level: If your total level is "L", you can only control Undead and parasites that each are CR L/3 (round down) and lower CR.  The maximum number of Undead and parasites you can control at the same time is 5*L, where "L" is your total level, and double this count if you are a Cleric OR multiply by 10 if the DM rules you are a Necromancer.  For example, a 12th level Necromancer can control 5*12*10 = 600 Undead & parasites at the same time by using this item. 

By using 1 action (to blow this whistle) and expending 1 charge, you cast a homebrew "Command" spell that works similarly (yet not identical) to a modified** "Dominate Person" spell on all Undead that are within 30' +5*L' radius of this whistle, where "L" is your total level or CR.  The targets (victims) of this spell get a WIS saving throw vs your "spell save" DC+2.   Creatures/Undead that are immune to "Charm" save at advantage vs this item's Command spell. Creatures/Undead that are resistant to "Charm" save at a bonus +2.

Command spell works like a modified version of the Dominate Person spell with the following differences and unique features: (A) This item's spell only works vs Undead and parasites/creatures that are parasitically linked to you; and (B) You don't need to concentrate because your most recent command that you speak to the Undead that failed their WIS save vs this spell will be under your command for the duration of this spell.   This spell works telepathically, so the undead can be blind and deaf and they will still "hear" you telepathically; and (C) The duration of this spell is 1 minute if you are 5th-6th level; or 10 minutes if you are 7th-9th level or higher; but the duration is 1 hour if you are 10th-12th level; yet the duration is 8 hours if you are 13th-15th level; but the duration is 24 hours if you are 16th+ level.  Controlled (dominated) Undead get another saving throw vs this spell if it takes damage.  If you or a controlled (dominated) Undead moves outside the "radius" of this item's Command spell, it remains under your control but beyond this spell's telepathic range ; and that means you cannot give that Undead a new command, but it will continue to follow your last command that you gave it when it was within the radius (range) of this spell. 

This item's charges replenish after you take a Long Rest.  Being attuned to this item makes you immune to Charm, including all Dominate spells, and you are immune to the effects of this item's Command spell. 

Notes: place this whistle in your mouth and blow while concentrating, provided you are at least 5th level and a Paladin or Cleric or Wizard or Sorcerer.