Death Slaad
A massive slaad capable of spellcasting, summoned from Trogeth's journal during the "test of strength." The creature tore itself free of the earth, shrugging off blows and retaliating with necrotic force.
The Death Slaad cast Fireball, engulfing the camp and incapacitating Aldric and Kouzlo. Madam Helena barely remained standing as the monsters collapsed back into smoke, drawn once more into the book.
Whether the battle occurred fully in the Material Plane or was imposed through Shadowfell perception remains unclear. What matters is that the wounds felt real - and so did the scrolls of Revivify that appeared afterward.