This is - or was - the skull of a human bard called Yorick. Yorick was a very capable bard and adventurer, trained in the College of Lore and frequently engaged for performances in high society. 

Yorick's adventuring came to an end over 300 years ago when he was engaged to perform for the Earl of Cowbridge, a powerful necromancer. Yorick stumbled across the Earl's secret ritual dungeon (Yorick will not elaborate why he was in the fourth level of undercellars behind a rack of aging cheeses, rather than the opulent music room in front of the audience he was meant to be playing for). For the embarrassment caused by ruining his party, and in order to keep his secrets, the Earl decided to make Yorick part of his ritual equipment, and had the bard decapitated, his skin flayed for bookbinding and bones boiled down to provide a base for necromantic potions. Yorick's death was avenged quickly by his friends, and his skull recovered. 

Yorick's spirit now inhabits his skull. Having been avenged and being freed from sins of the flesh,  his main aim in his afterlife is to keep travelling and to experience new things, since eternal rest sounds very dull.  While he can move by floating (speed 15ft), many people are disquieted by a floating skull, and so he prefers to attach himself to an understanding group of adventurers. In return for passage, a good view and not being put in a bag of holding or similar extradimensional plane, he is happy to divulge his experience (often at length and with a dated taste in jokes) and advice. He will require an assistant with experience in arcane spellcasting to properly orient and interpret his advice. A previous party constructed a fine leather harness for him so that he could be easily attached to a bag or staff, which makes him very happy.

Yorick has an AC of 13, 50 hit points (Yorick can only be knocked insensible and will revive after a long rest. He can only be killed by a disintegrate or wish spell). He has a strength of 1, dexterity of 18, constitution of 10, intelligence of 18, wisdom of 12 and charisma of 12 (being a floating skull has rather damaged his charisma). He has expertise in arcana, history and religion, though his answers are frequently abstruse. He is also happy to assist with performance, deception, intimidation and persuasion checks, providing advantage to rolls. With his assistant to point him in the correct direction, he can also cast bard spells up to 7th level which do not require material or somatic components. Yorick's magic has 8 charges renewed on a long rest - though Yorick doesn’t sleep and prefers to spend his nights chatting about the good old days with the watch keepers. He has darkvision to 60ft and truesight to 15ft, and will whisper his observations to his assistant.

• Vicious Mockery - at will
• Faerie Fire - 1 charge
• Healing Word - 1 charge 
• Knock - 2 charges
• Dimension Door - 4 charges
• Geas - 5 charges
• Teleport - 7 charges (note, familiarity for casting Teleport is Yorick's familiarity, rather than the assisting party member)

It it is worth noting that, as a sentient and autonomous floating skull, if Yorick grows bored with the party's direction, or feels he has been mistreated, he will place a geas on the party members at 7th level to forget his existence and make his way into the night to find a new team. Also, given the nature of his demise and afterlife, Yorick is exceptionally hostile to necromancers. He will do his utmost to ruin their day, and should he fall under the control of a necromancer and unable to escape, he will force himself to detonate, doing 10d10 piercing and 10d10 fire damage in a 60ft sphere, and a further 5d10 piercing and fire damage to targets up to 120ft away.

Notes: Expertise: Arcana, Expertise: History, Expertise: Religion, Advantage: Deception, Advantage: Intimidation, Advantage: Performance, Advantage: Persuasion, Sense: Truesight, Sense: Darkvision, Arcane spellcaster with a spellcasting modifier of 15+, Healing, Teleportation, Buff, Utility, Exploration, Sentient