A good place to collect this scroll is the basement of the Wizards' Guild in Yanille. The zombies are in a cage and need to be fought with magic. Bring runes for combat and telegrab. Another good place is Armoured zombies, which requires the Defender of Varrock quest. This scroll can also be obtained from an Undead Lumberjack which can be found in the Temple Trekking / Burgh de Rott ramble Activity. However it's extremely rare and it may take a long time before you receive it as drop.
Bring this scroll to Larxus in the basement of Champions' Guild to fight the champion in Champion's Challenge distraction and diversion. You may not use Magic attacks to fight the zombie. After you killed him, you will receive 240 Constitution and 240 Slayer experience.
- For an exhaustive list of all known sources for this item, see here.