You cannot activate the effect of “Exiled Force” if it is the only monster on the field.
Netrep Rulings:
“Exiled Force” may activate its effect immediately upon summoning before the opponent may respond to the summoning.
Tributing itself is a cost of using “Exiled Force’s” effect.
“Exiled Force” has an activated effect and is Spell Speed 1.
Netrep Q&As:
Q: Since “Exiled Force” can be summoned and tributed before the opponent can respond with something other than negating “Exiled Force,” can you chain “Torrential Tribute” to the effect of “Exiled Force?”
A: Yes you may. bishop, 12/15/2003
Q: Can “Exiled Force” be the target of its own effect?
A: No. When you activate the effect of “Exiled Force” you offer it as a Tribute. It will no longer be on the field to target with its effect. Curtis Schultz, 01/17/2004
Q: If “Exiled Force” is summoned to the field while there is a face-up “King Tiger Wanghu”, will the controller be able to activate the effect of “Exiled Force” before it is destroyed by the effect of “King Tiger Wanghu”?
A: This is similar to “Exiled Force” and “Mysterious Puppeteer.” The effect of “King Tiger Wanghu” will activate automatically when “Exiled Force” is summoned. Due to this, “Exiled Force” will not be able to activate its effect. Curtis Schultz, 02/09/2004