Gambling addiction

{r}{2}, roll a d20, divide that roll in half, add that to your life total (if it lands on odd round up)
1-5 all cards under your control cost 1 less mana (stacks)
5-10 all your lands count twice
10-15 all creatures under your control
gain double strike
15-19 target player exiles all lands
20 cut off target 3/4 of target players life total