From Halopedia, the Halo wiki
The Lich is a type of heavy dropship utilised by the Covenant and its remnants. Much larger than their contemporaries such as the Spirit, the Lich is capable of housing dozens of troops, extremely heavy weaponry, and can even transport a Phantom. When deploying troops, the Lich employs its Gravity lift to safely drop dozens of soldiers to the battlefield. The Lich itself is armed with an Unknown Energy Beam weapon that can heavily damage the toughest of foes, such as a Mammoth.
The Type-56 Lich was employed by the Covenant as its heaviest class of dropship. They were utilised in the Human-Covenant War but were so destructive that oftentimes, UNSC forces deployed against them suffered near-total annhihilation, limiting information about them readily known to the UNSC. In the post-war period, they are employed by Covenant Remnant factions such as Jul 'Mdama's Covenant and the Swords of Sanghelios.