If you're playing the Steam version or you've got the Premium Pack, the Zombie Hydra will be lurking at the northern end of the tomb, right beside Mike. You'll need the Winged Boots to reach it - and when you do, be prepared for a tough fight.
Mike: Bring 1 Hamburger, 1 Chips, and 1 Softdrink
REWARD: 1 Bottled Darkness, 1 Dark Rune, and 1 Dark Matter
Undying Monolith: Two waves.
Wave #1: Three Sacred Runes and two Undying Monoliths
Wave #2: Two Frost Wraiths, one Evil Worm, and one Evil Tail
Sacred Rune: Three waves.
Wave #1: Two Sacred Runes, two Frost Wraiths, and one Undying Monolith
Wave #2: Two Sacred Runes
Wave #3: Three Undying Monolisths and two Frost Wraiths
Zombie Hydra: Three Zombie Hydras (only appears in Premium Pack or Steam version; need Winged Boots to fight)