EDIT: The names of the tablets are now added, with the courtesy of Halder to list them
To get this achievement you have to collect 11 Mysterious Tablets. Also known as Treasures of Britain. (They are only in England.)
1) 8 tablets are in cave puzzle locations
*** Spoiler - click to reveal ***
In order of the linked video:
Hilt - Grime’s Graves – East Anglia
Horn - Old Cellar - Essexe
Chessboard - Cavern of Trials - Cent
Cauldron - Santlache Mine - Suthexe
Crock and Dish - Wiccan’s Cave - Eurvicscire
Whetstone - Deoraby Spar Cavern - Snotinghamscire
Coat - Wocig - Hamtunscire
Mantle - Red Lichen Cavern - Hamtunscire
2) 3 Zealots carry a tablet
*** Spoiler - click to reveal *** Chariot - Heike - Zealot lvl 250
Halter - Hrothgar - Zealot lvl 280
Knife - Woden - Zealot lvl 220
To kill Heike or other shield Zealots I recommend getting the skill: Brush with death. This will make the fight something to endure and to focus on rather than autopilot fighting. They are the trickiest Zealots of the three so go in with full rations and arrows and some adrenaline. You could also apply Tyr's Kick but I could only sometimes be fast enough to get a Stomp in
To defeat any other type of Zealots I recommend a light bow to hit their weak spots and then a stun / finisher to deal a lot of damage. Wielding a hammer as a main you can knock them down and do Stomp attacks. By then your adrenaline should've charged up and you can spam Dive of the Valkyrie to kill them. I was level 130 when I killed all Zealot's on easy attack difficulty
3) Excalibur itself is located in Myrddin’s Cave in Hamtunscire
Or just follow this 37min video that covers it all, except for my awesome Zealot strats