If your motor/prop really does draw 900 watts, that would require around 80 amps of current. If you try to run it on that battery, you will only get what the battery is capable of supplying. You will most certainly ruin that pack if you overdraw it like that. Whether it will catch on fire or not???? It all depends on the quality of the pack. It will most likely puff really bad, and then die a miserable death.

Even though a motor is rated for 900 watts, that doesn't mean that's what it's going to produce. That all depends on the battery, prop, Kv rating, etc. The power system will only draw what it needs depending on all these factors.