A teleport focus can be built as the centrepiece of the Portal Chamber in a player-owned house. It requires 50 Construction, a saw and hammer and 2 limestone bricks to be created. It is the first centrepiece able to be built in the Portal Chamber, the second being the Greater focus and the third being the Scrying pool. A teleport focus is required to build portals. Once you've constructed the portals and directed them to a location, you can remove the teleport focus if you wish to do so. Note, however, that you will need to build one again if you want to change the location a portal teleports to.