Not much is known of the inter-dimensional Empire of Tandia. Here are some observations:
1) Tandia consists of a wide variety of races. One of the first incursions took place in the UK near Oxford, when individuals calling themselves "recon group Wyvern" exited a dimensional portal. The group consisted of an elf, a mythical dwarf, and a troll. Among the humans of the group could be seen a caucasian and two of apparent East Indian descent. Ireland was attacked primarily by the Fir Bolg (more on them below) and California was assaulted by "mer men" (also see below).
2) Tandia may be more of a loose confederation of states than an actual empire. While the Fir Bolg were tasked with attacking Ireland what appeared to be futuristically armed Imperial Romans attacked an area near Chicago. Every major dimensional incursion was handled by a different sort of invader with their own level of mystical and/or technological capability.
3) Tandia cannot open dimensional portals any place they please. Or at least they cannot open large ones. Evidence has shown that every site where a portal opened contained some mystical history from one culture or another.
4) The Earth is still under surveillance. In December '01 an extra-dimensional craft crash-landed in Greenland. The ship was from an alternate dimension where the inhabitants of the British Empire seem to be Chinese and possess technology different from this Earth's. These British have proven to be quite open and cooperative and claim their Earth is allied with an "Inter Dimensional Confederacy". More importantly, they claim their ship was blasted into this dimension by a warship hovering on the edges of our own dimensional plane. Who this ship belongs to, be it the Tandians or some other power, is not known at this time.
5) Agents are still among us. High Justice was taken down before the invasion, suggesting advance agents were on Earth well before Tandia attacked. And it is known invaders stayed behind and disappeared. Sometimes they are caught, but officials in most nations that were attacked have admitted they have not caught them all.
6) The Tandians do sport a common thread among their units. The various images of a dragon could be seen in many cases; the robes of the Golden Papacy, the banners of the Roman Imperialate, the armor of the Mermanus. Units were called "Wyrm" "Wyvern" "Kommodo", etc.
Below is a more detailed look at the known factions of the Tandians:
THE FIR BOLG: Large and fierce, the Fir Bolg are crimson-skinned, ape-like humanoids armed with flintlocks and bad tempers. They swarmed through Ireland and it was only due to the combined efforts of the Irish Army, British Army The Vindicators, the Faey Nation and a group of independant paranormals that drove them back.
The Fir Bolg are long on courage, short on intelligence. They are superstitious and can be easily led. They are foot soldiers, little more.
THE GOLDEN PAPACY: A twisted form of the Catholic Church, The Golden Papacy manipulated and led the Fir Bolg in their attack on Ireland. The Priests wore strange masks and flowing golden robes. An infiltration of a Tandian "way station" revealed members of the Papacy smoking marijuana, with attractive nuns dressed in lewd versions of habits and wearing make up.
The members of the Papacy wield powerful magic, but they are limited and could be killed through conventional forces. It also seems Earth based religions may be used against them; Patrick Regan, a staunch Catholic and lethal sniper, fired a bullet that pierced a Golden Papacy priest's mystic shields. He said he had asked a local Catholic priest-a good man who was much loved by his congregation-to bless his bullets and pray for him. The evidence is still circumstantial but the possibility of using this as a weapon is under way.
THE ROMAN IMPERIALATE: Much like the latter day Roman Empire, the Imperialate is composed of many human races rather than made up purely of Italian stock. Prisoner interrogations reveal the Imperialate does indeed date back to Imperial Rome, but instead of falling to barbarian hordes said hordes were themselves the victims of a series of plagues that destroyed them before they could destroy the Romans.
Even so, the Romans would have stagnated if not for the intervention of the Tandians, who elevated their technological level in exchange for fealty and to act as shock troops.
Vindicators Behind The Veil