The Life Guardian can be summoned by creating a 5x5 ring made of Wizard's Chalk, placing Warding Candles on the center of each side outside the ring, and then killing a baby Villager on the night of the full moon.
The Life Guardian is unique from the other bosses in that it is the only one that will spawn Elementals to fight for it, instead of attacking directly. While its minions are alive, the Life Guardian is invulnerable.
Arcane Compendium· Crystal Phylactery· White Chalk· Blue Chalk· Red Chalk· Keystone· Lightning Charm· Life Ward· Mana Cake· Player Journals· Spell Parchment· Warding Candles· Wizard's Chalk· Workbench Grid Upgrade
Air· Arcane· Earth· Ender· Fire· Ice· Life· Lightning· Nature· Water
Hell Cow's Horn· Nature's Scythe· Winter's Grasp
Ender Boots· Magitech Goggles
Circlet· Armor· Leggings· Boots
Antenna of the Fire Guardian· Earth Armor· Water Orbs
Sigil Stand· Sigil of Gentle Rains· Sigil of Nature's Bounty· Sigil of The Felled Oak· Sigil of The Flat Lands· Sigil of Butchery· Sigil of Progeny· Sigil of Containment· Sigil of Interdiction· Sigil of The Light· Sigil of The Lunar Tides· Sigil of The Packed Earth