The 5E Warlock Invocation 'Frost Lance' (from Unearthed Arcana) says:
You draw on the gifts of the Prince of Frost to trap your enemies in ice. When you hit a creature with your eldritch blast cantrip one or more times on your turn, you can reduce that creature’s speed by 10 feet until the end of your next turn.
Does this mean that if I hit a creature twice in one round, his movement penalty is -20'?
No, you can only reduce a specific creature's speed by 10 feet.
When you hit a creature with your eldritch blast cantrip one or more times on your turn, you can reduce that creature’s speed by 10 feet until the end of your next turn.
This ability triggers when you hit a creature with eldritch blast one or more times, not whenever you hit a creature with eldritch blast. No matter how many times you hit a creature, you will still only have triggered this feature once.
Yes and no. By RAW the invocation states "one or more times on your turn" and "Until the end of your next turn."
By taking the two sentences above we can deduct that on turn 1: You hit the enemy with 1, 2, or eve 5 blasts(doesn't matter); Their speed is reduced by 10ft. On the next turn you could indeed hit them again, reducing their speed by a total of 20ft... However, by the end of that next turn they'll have lost the speed reduction from the first attack (thus becoming -10ft once more).
So "yes" while you can stack up to a -20ft penalty, and "no" it'll effectively only be -10ft penalty at a time. Hope this helps :)
