During Hereward's large and hugely effective rebellion at Ely 1069-71, because of which William took personal command by the vast but murky marshes as his men suffered heavy losses in Hereward's cunning ambushes, the Norman king got more desperate...and hired a witch!
Odd action for a supposedly 'Christian' king who had invaded a 'corrupt' nation in order to bring the English church 'back into the fold' (but one which a visiting Papal legate in 1061-2 found nothing 'corrupt' about?)?
One of William's commanders, Ivo Taillebois, suggested using the services of 'an old woman' (witch) who could cast spells and curse the valiant defenders, so hopefully crushing their courage and will to resist. Some others who had survived the violent skirmishes dismissed Ivo, but William accepted the idea and had her brought to him.
With the Norman wooden assault 'towers' ready (from which to fire crossbows from, or espy their numerous but hidden enemy), the witch mounted the foremost one and began 'cursing' Hereward's men and tried to provoke them.
She also reputedly bared her buttocks at them - reminiscent of the defender at Exter in 1067 who bared his genitalia at the Normans under the bastard himself, and farted at them (he may have soon regretted that- there were brutal reprisals when the town fell- just as those jeering but hapless townsfolk at Alencon did two decades earlier).
However, Hereward and a retinue of men were reputed to have disguised themselves and mingled with the English fishermen who had built the causeway and when ready, they threw off their disguises and fired flaming arrows at the unused heaps of wood on which the towers stood, throwing the Normans into disarray- and the towers into pillars of fire.
Many Norman troops, maybe in the mindset of witches and spells, fled in terror at the sight of raging fire spreading, and the crackling sound it made- chased fiercely by Hereward's men who slew them with swords/ bows and various missiles(with the English knowing the trackways well, William could do little to rally his men).
The witch, choking and terrified, fell headlong from the flaming tower and broke her neck. As the flames whipped by the wind into ferocious mini-storms, the towers collapsed into the tinder swamps and spread quickly in a wide area. Fire now engulfed a large area of the Fens. Maybe this is when William Malet died?
Of course, as the siege took it's toll, some of the fearful Ely monks led the Normans into the treacherous marshes...and betrayed Hereward's men.