how about as you said before a nanite storm but it could be like the one in the book "Prey" by Michael Crichton. If your not familiar it was about a rogue nanite swarm who became self-sufficient and started hunting (they were called nano-machines). It would be easy to make as it could be a small version of the sandstorm but coloured black. In the book the swarm attacked and killed, in PS it could slowly strip away armor points then slowly take off 1 life point every so often. This would encourage people to take the fight inside and push forwards into the base quicker. The swarm would fit in with PS as it could be the product of a out of control VS experiment in evolution of technology
