Ramuthra was a Greater Sprit summoned by Simon Lovelace and his co-conspirators during their attempt to stage a coup against the senior magicians of the British Empire.
Ramuthra's sole appearance was in "The Amulet of Samarkand". It was summoned, contained and then temporarily bounded into a holding pentacle using the combined efforts of four high level magicians before it was summoned by Simon Lovelace with the Summoning Horn into the large, hidden pentacle holding the majority of the British Government.
Lovelace then commanded it to destroy the Government Magicians while under the protection of the Amulet of Samarkand, which contained a spirit at least as powerful as Ramuthra. Despite the efforts of the most powerful spirits present (including a very high level Afrit, which was destroyed when it drifted into a rift created by Ramuthra's summoning), its powers negated every single attack sent against it.
Ramuthra was sent back to the Other Place after Nathaniel succeeded in stealing the Amulet of Samarkand from Lovelace's neck. Without the protection of the Amulet, Ramuthra grabbed a hold of Lovelace and consumed him, after which Nathaniel broke the Summoning Horn and spoke the words of dismissal. Ramuthra is the second most powerful Spirit seen in the Trilogy, surpassed only by Uraziel.
As a Greater Spirit from the heart of the Other Place, Ramuthra's presence alone was enough to warp the elements and change the physical states of objects and living things that came into contact with it. According to Bartimaeus, this was in part due to the fact that Ramuthra's nature brought with it the chaotic nature of the Other Place.
Ramuthra's presence had such a disruptive effect that even magical spells cast by extremely high-level afrits simply vanished or lost all their power, merely warping into sludge-like waste. As such, Ramuthra was a veritable juggernaut, almost untouchable by man or spirit.
Ramuthra did not have a fixed form, and as only given shape by the vague outlines that were caused by the shimmering and fluctuating planes of reality, for his very existence causes the laws of nature to rebel.
It apparently moved sluggishly, causing Bartimaeus to speculate that it had been scarcely (or never) before summoned into the earth, and that it was unused to the laws of the physical world.