The very first encounter that the imperium had with the tyranids was on a outer rim planet in the south-eastern part of the galaxy about 60 000 light years from earth. There was a base located there whose purpose was to scout the outer finged planets, but all the planets which had been scouted hundreds of years ago which had supported life were all barren.
A scout ship passed by Tyran every three to six months delivering supplies and going on to scout more planets. The ship that had just passed a month earlier was Investigation when the base detected a strange cloud in the Tyran star system. When a ship was sent to investigate it was crippled, but they discovered the cloud was made up of thousands of pods and ships.
Tyran's base was located on an island surrounded by ocean at the tip of an ancient volcano ring. The cloud surrounded the planet and disrupted the warp so no one could send out messages. The first attack was driven back, but only twelve creatures amoung the thousands were killed.
The main attack was immense and started with the sky turning black from all the pods dropping. The pods landed in the sea around and creatures came out of all the pods and soon the base was overrun. These monsters tore through the base walls like paper and went through armour with the greatest of ease. Varnack the commander of the base sent out a transmission and hid a codex on these beast.
The Inquistor and its ship received the last transmission from the planet about six months later and arrived at the planet a year later. When they found the codex the ship tried to warn the planets but the warp was still disrupted by the Tyranids and when the ship went to Thandros all the planets had been overrun with ease by the Tyranids. On Thandros they were finally able to send a warning to all the planets about the Tyranids.
The Tyranids next headed for Macragge. The Ultramarines were able to prepare for the invasion and for weeks the planet prepared with ships, tanks, and troops arrived at the planet. Finally the Hive Fleet arrived a major battle happened soon after. At first the Tyranids were winning but then one of the main Hive ships was crippled and the attacks became uncoordinated but the Tyranids kept fighting a paying in their blood. The fight was ferocious on the front lines and the Tyranids overrunning every perimeter and the Ultramarines kept retreating until the Bakka fleet arrived and turned the battle and the Ultramarines slaughted the fleet.
For years after the battle the Techno Magi of Mars classified the different types of Tyranids and artifacts, but the scientist weren't able to find out very much about their origin or how they came into being.