implacable:

incapable of being pacified.

impost:

1. A tax or a similar mandatory payment.

2. The weight a horse must carry in a handicap race.

3. The top part of a pillar of a wall, usually projecting in the form of an ornamental molding, on which an arch rests.

incontrovertible:

indisputable; unquestionable.

invidious:

tending to provoke envy or ill will.