As you already know, Donald Trump is a pants-on-fire category liar

Sources:
Donald Trump's Pants on Fire claim about the estate tax, small businesses and farms.  (PolitiFact, 9/28/2017)

The estate tax was first established in 1916 to offset a decline in tariff revenue caused by the first World War, according to the Congressional Budget Office. Congress has tinkered with it many times throughout the years.

Policy Basics:  The Estate Tax.  (Center on Budget and Policy Priorities, 8/14/2017)

The federal estate tax is levied only on the portion of an estate’s value that exceeds the exemption level, minus deductions, such as for charitable giving. Largely because of its generous exemption levels, only the wealthiest 0.2 percent of estates pay any estate tax, and typically at fairly moderate rates. The tax also affects very few small farms and businesses.

Related post:
Tax relief for the very, very rich.  (11/18/2017)