Warning: The following text contains spoilers relating to the plot of the Factions Campaign.
The Spear of Archemorus does not have to be held to charge the effect.
It is best that the wielder not be a monk or any caster standing at the back. The spear charges in levels as creatures die nearby. The maximum level of charge is five (5). The Spear charges very fast when used in a party with a minion bomber. The minion bomber lets his minions die, charging the spear very quickly.
When dropped, the spirit of Archemorus will be seen emerging from the spear and then shortly after, the damage that was stored in the spear will be released to nearby foes. A maximally charged spear will do 300 damage to nearby ordinary foes and 1900 damage to the Zhu Hanuku. While the range is still unknown it is believed to be about the size of the aggro circle.