Stone Guardians

The Stone Guardians are two magically enchanted statues guarding the entrance to the Shrine of the Codex on the Isle of the Avatar. They basically look like huge, winged humans that look at each other while kneeling. They only allow someone on a sacred quest to pass through to the shrine. Everyone else is turned away.

It is unknown who had erected them, although the Great Council is a possibility. However, the magic barrier surrounding the Codex of Ultimate Wisdom clearly doesn't originate with them, since Naxatilor had put it on the Codex in ancient times.

The Stranger was able to pass in Ultima IV, since the Quest of the Avatar counted as sacred quest.

History
The Stone Guardians were first encountered in Ultima V. They didn't allow passage unless the Avatar got a sacred quest from one of the eight shrines of Britannia. After all quests were fulfilled, the Guardians blocked the entrance. Strangely enough, should Lord British resurrect the party, the Guardians were visible as well.

In Ultima VI, the Stone Guardians still fulfilled their function. This time however, they also blocked entrance after speaking to the shrines, since no quests were given. Only after getting approval from the Shrine of Singularity did the Stone Guardians allow the Avatar to pass and reach the Codex. It was also shown with Bolesh that trying to force the way through anyway would lead to horrible injuries.

They were still found in the same place in Ultima VII. However, since the Codex had been returned to the Ethereal Void, the protective magic was gone and the Stone Guardians had become inactive, allowing anyone to come and go as they pleased. 

Ultima Online
In Ultima Online, the Stone Guardians don't exist, as the Codex had never been brought to Britannia. However, on Ter Mur, a very similar set of guardians exist in the Tomb of Kings, who also only let people pass who are on a sacred quest, which can be gained from the Shrine of Singularity.

However, they don't allow entrance to the Codex. Instead they allow acces to the gate that leads to the Abyss and therefore, Britannia.